centre the embryonic fillet takes its rise. In many cases the ridge of the blastoderm bears two tubercles, from each of which an embryonic fillet is produced, and the further development of these gives rise to double embryos of various kinds. It appears from a statement of M. Lereboullet, that the formation of these monsters may be determined at pleasure by placing the eggs in unfavourable conditions for development.

M. Lereboullet describes the formation of several varieties of these double monsters. 1. In some instances two tubercles were produced on the margin of the blastoderm, from each of which proceeded a fillet furnished with a dorsal furrow, forming two embryos adhering to the marginal ridge. Soon afterwards the divisions of the vertebræ made their appearance, the external ones having their ordinary form and dimensions, whilst the internal gradually became confounded, passing from the body of one embryo to that of the other, thus causing the partial amalgamation of the two embryos. In this manner a double fish is formed, arising from two primitive germinating points produced upon the blastodermal ridge, so as to become partially joined; it has therefore two separate bodies with a common tail. A specimen of this description, in which the fusion of the double embryo had extended to about the middle of the body, lived for four days after exclusion from the egg.

2. In other eggs the blastodermal ridge gave rise to a long and broad fillet terminated anteriorly by two rounded lobes. Two parallel furrows made their appearance in the fillet, and soon exhibited the vertebral divisions, whilst the anterior lobes acquired a determinate form, and each produced two ocular vesicles, constituting an embryo with a single body and two distinct heads. In these cases however the duplicity was transitory; the two heads soon came in contact, and became soldered together in such a manner as to form only a single head. M. Lereboullet has observed these phænomena in about fifteen instances, but could never ascertain in what manner the fusion of the two heads was effected. In some cases the two heads appear to remain distinct.

3. One of the most remarkable monstrosities is that presented by some embryos with a single head, two separate bodies, and one or two tails. These are formed in the following manner :the ridge of the blastoderm, which has the form of a gaping button-hole, produces a single cephalic tubercle, but the formative process goes on in the whole circumference of the margin, each half of which acquires a chorda dorsalis and a nervous cord, and soon exhibits the divisions of the vertebræ. When the cephalic tubercle is short and only gives rise to the true head, each of the two bodies is furnished with two auditory capsules, Ann. & Mag. N. Hist. Ser. 2. Vol. xvi.

4

two pectoral fins and a heart; but when this tubercle is more elongated, the anterior part of the body is simple, and bears two eyes, two auditory capsules and a single heart, and the body is terminated posteriorly by two short branches.

4. This organization of the ridge of the blastoderm into a double embryo seems to explain an extremely curious form which only occurred once in M. Lereboullet's observations. This consisted of a simple embryo, bearing on the right side of its body a small tubercle, directed backwards, and terminated by an auditory capsule and an active heart. The production of this form is accounted for by the resorption of the parts of the body posterior to the heart in one of the embryos, and M. Lereboullet has witnessed the complete disappearance of one of the bodies

in other instances.

5. In another egg the ridge of the blastoderm exhibited two contiguous tubercles, of which one had the ordinary form of the cephalic tubercle, whilst the other was smaller and irregular. The former alone acquired a furrow and gave rise to an embryo, on one side of which the smaller tubercle was borne.

6. Occasionally bodies furnished with three heads make their appearance. One of these is described by M. Lereboullet. It was a double embryo, composed of two bodies united behind, but quite free in front. One of these bodies was of the normal form; the other bore two heads, of which that on the left was of the normal form and furnished with two eyes, whilst that on the right only bore the right eye, the union of the two heads being effected at the point where the left eye ought to have been. This singular embryo was still in the egg when described; it had two hearts, one common to the two principal bodies, situated at their bifurcation, the other placed in the angle of union of the two heads.

M. Lereboullet explains the formation of this anomalous creature in the following way. He considers that two fillets have been formed, one of which has been terminated by two cephalic lobes and acquired two furrows (as described under 2.), whilst the other has remained simple. These two embryos have then united posteriorly (as under 1.), producing an embryo with one tail, two bodies and three heads.

7. An extraordinary result was obtained when the development of the eggs was retarded by a considerable diminution of temperature. In this case the ridge of the blastoderm produced no embryo, but contracted gradually like the opening of a bag, its substance becoming condensed and forming a mammillated tubercle projecting from the surface of the vitellus. This tubercle continued living, rose more and more from the surface, acquired a lingulate form, and at last constituted an elon

gated body, narrowed in front, divided transversely into vertebral lamellæ, without dorsal cord, or sensitive organs, but furnished with a heart, of which the contractions were sometimes very lively.

These observations, as M. Lereboullet well observes, appear to prove that the generally received opinion that double monsters are produced by the fusion of two embryos is perfectly correct, whilst that which attributes a separate vitellus to each of these embryos is not founded in fact. He considers that his observations, with those of Valentin, show that there is only a single germ, but that this, by becoming developed in two directions, instead of one as in normal cases, gives rise to two more or less distinct embryos. In his opinion the ridge of the blastoderm (bourrelet blastodermique) plays a most important part in the formation of the embryo, and in fact constitutes the "true embryonic germ, which is always simple and single, like the vitellus which is covered by the blastoderm, but when its development is deranged from its regular course, is capable of vegetating like the substance of which the bodies of polypes are composed, so as to produce various forms, which however, in their subsequent development, always show a distinct tendency to return to the original type of the species."

PROCEEDINGS OF LEARNED SOCIETIES.

LINNEAN SOCIETY.

December 5, 1854.-William Yarrell, Esq., V.P., in the Chair. Mr. Ward, F.L.S., exhibited two sets of specimens of Asplenium lanceolatum, from Jersey, both found growing on disintegrated sandstone, exhibiting a striking difference between the growth of the same species on an open sunny bank and in dense shade.

Mr. John Hogg, F.R.S., F.L.S., exhibited some scales, and a piece of the scaly covering which was cut from the back of a large fish found in the river Tees, in September of this year. He stated that two fishermen observed a great fish-such as they had never before seen-left by the tide on a sand-bank, in the estuary of the river Tees. They described it as having the head of a salmon, with the back-fin like that of a perch, erect, and somewhat spiny, and the tail spreading and much curved. The colour they did not mention, except that of the back, which was represented as being of a purplish-black. They likewise particularly observed some large scales on the front of the fish near the gill-covers, one of which Mr. J. Hogg also exhibited, and which is of a very strong bony texture. From the account of this fish so given, Mr. Hogg conceived that it could only have been a large Tunny (Thynnus vulgaris of Cuvier), which had been stranded whilst in pursuit of herrings or other small

fishes. MM. Cuvier and Valenciennes in their Hist. Nat. des Poissons,' tom. viii. p. 57, separated the Tunnies from the Mackerels (Scomber), in consequence of the "remarkable disposition of the scales on the thorax, which are larger and more unpolished than the others, and form around that part a sort of corselet?" They also describe the corselet of the Common Tunny thus:- -"Le corselet, c'est-à-dire cette portion du tronc couverte d'écailles plus grandes et moins absorbées dans la peau, est considérable." (p. 62.) This, however, would seem to lead to an incorrect view of the smaller scales on the back, which are, as it were, enclosed between two skins, and are placed in a somewhat imbricated manner, resembling the arrangement of the slates upon a roof, and cannot properly be termed "absorbées dans la peau." Mr. Hogg showed the nature of the piece of the external covering which the fishermen had cut off the back; the outside skin being of a dark, or nearly black colour, and of a coriaceous substance. The white scales, imbedded in it, are similar to those forming the corselet near the gill-covers, but they are much smaller, and so closely placed by one overlapping the other, that they constitute a perfect defence against nearly every kind of danger. The piece so cut off the back of the fish, which was done with some difficulty, resembling a portion of a shield, would seem to be (from the close disposition of the osseous plates or scales enclosed between two skins) proof against large shot, or even a ball from a musket. Both the external large plate-like scales forming the corselet, and those, arranged in a slate-like manner, between the skins, are of an irregular, but somewhat round, or oval, shape. When seen from the outside of the external black skin, the plain parts between the extremities of four scales, present, by the overlapping of their edges, a somewhat regular appearance, and are of a nearly uniform size, viz. about half an inch in length, by a little more than one-eighth of an inch in width, and they thus exhibit much of a diamond pattern.

Mr. Hogg observed, that ichthyologists seem not to have described this remarkable protection, presented by the thick skins, and strong bony interlaminated scales, which is evidently a beautiful provision of Nature to defend these fishes from the attacks of their enemies, and especially those of their greatest foe, the sword-fish.

Read a paper "On Decaisnea, a remarkable new genus of the tribe Lardizabaleæ," by J. D. Hooker, Esq., M.D., F.R.S., F.L.S., and Thomas Thomson, Esq., M.D., F.L.S.

The small family of Lardizabaleæ, which was first instituted many years ago by Mr. Brown, and chara cterized by the distribution of the ovules over the whole surface of the ovary, was afterwards admirably illustrated by M. Decaisne in a memoir in the Archives du Muséum.' Though the peculiar distribution of the ovules has always been justly regarded as the most striking characteristic of Lardizabalea, it is by no means the only peculiarity of the order, which may be distinguished from all its near allies by a considerable number of very striking characters, sufficiently proving the distinct

ness of the order even where the prominent characters of the insertion of the ovules and the digitate leaves are absent. This is remarkably the case in the plant to which the authors of this paper called the attention of the Society, and of which a figure was placed on the table.

This interesting plant, which was originally discovered in Bhotan, by Mr. Griffith, is briefly referred to in his 'Itinerary Notes,' under the name of Slackia insignis, a name evidently imposed on a conviction that the many striking characters which it presents warranted the establishment of a new genus, to which, however, no characters were assigned. That name having (before the publication of these 'Itinerary Notes,' in which it was only a manuscript designation,) been applied by Griffith himself, in his Essay on Palms,' to a genus of that order, the authors proposed to designate the plant now described, Decaisnea, after the distinguished monographist of the group to which it belongs, as the two genera of Orchidea which have been so called have both proved to have had earlier names.

DECAISNEA, Hf. & Thoms.

Sepala 6, lineari-subulata. Petala 0. Stamina in flore masculo mona delpha, in hermaphrodito parva libera. Ovaria 3, lineari-oblonga Ovula placentis 2 filiformibus parallelis, suturæ ventrali approximatis sed ab eâ discretis, inserta, indefinita, anatropa. Folliculi pulpâ repleti. Semina indefinita, compressa; testâ lævi, nitidâ, crustacea, atrofuscâ. Frutex Himalaicus erectus; foliis alternis, pinnatis; inflorescentiâ racemosâ, terminali; floribus pallidè viridescentibus.

Decaisnea insignis is a native of the temperate parts of the Eastern Himalaya, at elevations between 8000 and 10,000 feet. As mentioned above, it was first discovered in Bhotan by Griffith. Dr. Hooker's specimens are from the interior of Sikkim. It is an erect shrub, with large simply pinnated leaves, and a nearly simple trunk, marked by large scars after the leaves fall away, as in many Araliacea. The nearest analogy is no doubt with the section Mahonia, of the genus Berberis, but the leaves of Decaisnea are soft and thin, not rigid and prickly as in Berberis. The flowers are arranged in elongated racemes, and closely resemble those of other Lardizabaleæ. Their colour is pale green, slightly tinged with purple towards the apex. The sepals are elongated to a subulate point, and there are no petals. The stamens on the male flower are monadelphous, and very like those of Stauntonia or Parvatia. In the fertile flower the stamens are free and very small, but the anthers always contain pollen, so that the flowers are rather polygamous than dioicous. The most remarkable character is exhibited by the ovaries, which, though externally not unlike those of Hollböllia, have the ovules arranged in a double series along two elevated lines, one on each side of the ventral suture, and not scattered over the whole surface of the ovary as in all the other genera. The same arrangement is preserved in the ripe fruit, which consists of three large follicles full of an agreeably-flavoured solid pulp, dehiscing along the ventral