Laramie Group I have had reference almost entirely to the invertebrate fauna, which consists, so far as the discussions are concerned, entirely of the Mollusca. This was not because the investigation of those subjects is more in the line of my special studies, but because being inhabitants of the waters in which the formations were deposited, they had a more direct bearing than any others upon the physical phases of the western portion of North America during the period that has been discussed, and, also, because neither the then existing vegetation nor the most important part of the vertebrate fauna was necessarily affected by at least those physical changes which caused an entire change of the whole molluscan fauna, both at the beginning and close of the Laramie period. The reptilian fauna of the Laramie period, however, assumes especial interest, because certain of its types, which extend throughout the whole vertical range of the group, are regarded as characteristic of Cretaceous age.

Notwithstanding the positive opinions that have been expressed by others upon the subject of the geological age of the Laramie Group, I regard it as still an open question. All paleontologists agree that the Cretaceous period extended at least to the close of the Fox Hills epoch; and the question is whether the Cretaceous period closed with the close of the Fox Hills epoch or with that of the Laramie period. The question might be extended so as to embrace the inquiry whether the true chronological division between the Cretaceous and Tertiary did not really occur within the Laramie period; but this, while not unreasonable, would per haps be inconvenient and unprofitable. That, according to European standards, the Dinosauria which are found even in the uppermost strata of the Laramie Group are of Cretaceous types is doubtless indisputable, and there also appears to be no occasion to question the reference that has been made of fossil plants which have been obtained from even the lowest Laramie strata, to Tertiary types. The invertebrate fossils, of the Laramie Group itself, as I have shown in other writings, are silent as to its geological age, because the types are either unique, are known to exist in both Mesozoic and Tertiary strata, or pertain to living as well as fossil forms.* Every species found in the Laramie Group is no doubt extinct, but the molluscan types have collectively an aspect so modern that one almost instinctively regards them as Tertiary; and yet some of these types are now known to have existed in the Cretaceous, and even in the Jurassic period. In view of these facts, together with those presented in the foregoing discussions, the following suggestions concerning the geological age of the Laramie Group are offered.

It is a well known fact that we have in North America no strata which are, according to European standards, equivalent with any part

* It is a fact worthy of consideration in this connection that a large proportion of the molluscan types of the extensive fresh-water deposits of Southeastern Europe are practically identical with some of those of the Laramie Group, and that European geologists regard those deposits as of Eocene Tertiary age.

of the Lower Cretaceous of Europe, but that all North American strata of the Cretaceous period are equivalent with certain portions of those of the Upper Cretaceous of that part of the world. That the Fox Hills Group is of Upper Cretaceous age no one disputes, the only question being as to its place in the series. A comparison of its fossil invertebrate types with those of the European Cretaceous rocks indicates that it is at least as late as, if not later than, the latest known Cretaceous strata of Europe. If, therefore, that parallelism is correctly drawn, and the Laramie Group is really of Cretaceous age, we have a great and important division of the Cretaceous represented in America which is yet unknown in any other part of the world. It is in view of these facts that, for purposes of general grouping of the strata of the Western Territories, the provisional designation of "Post-Cretaceous" has been adopted for the Laramie Group in the reports of this Survey.

It is well known that able American paleontologists regard the Laramie Group as of Cretaceous age, and this opinion is understood to be based upon the persistence of some vertebrate Cretaceous types up to the close of the Laramie period and the first known appearance of Tertiary types of mammals in North America, in the immediately superim posed Wasatch strata. It is not to be denied that these are important considerations, but the following, as well as other relevant facts already mentioned, ought to be duly considered in that connection.

With rare and obscure exceptions, no mammalian remains are known in North American strata of earlier date than those of the Wasatch Group that were deposited immediately after the close of the Laramie period. Immediately from and after the close of that period, as shown by abun dant remains in the fresh-water Tertiaries of the West, highly organized mammals existed in great variety and abundance. There is nothing to forbid the supposition that all of these were constituents of a Tertiary fauna, and many of them are, by accepted standards, of distinctively Tertiary types. If the presence of these forms in the strata referred to, and their absence from the Laramie strata immediately beneath them, together with the presence of Dinosaurians there, be held to prove the Tertiary age of the former strata, then was the Tertiary period ushered in with most unnatural suddenness. Sedimentation was, at least in part, unbroken between the Laramie Group and the strata which contain the mammalian remains referred to, so that the local conditions of the origin of all of them were substantially the same, and yet, so far as any accumulated evidence shows, those mammalia were not preceded in the Laramie period by any related forms. Such suddenness of introduction makes it almost certain that it was caused by the removal of some physical barrier, so that the ground which was before potentially Tertiary, became so, of paleontological record, by actual faunal occupancy. In other words, it seems certain that those Tertiary mammalian types were evolved in some other region before the close of the Laramie period, where they existed Bull. iv. No. 4- -9

contemporaneously with at least the later Dinosaurians, and that the barrier which separated those fauna was removed by some one of the various surface movements connected with the evolution of the conti nent. The climate and other physical conditions which were essential to the existence of the Dinosaurians of the Laramie period having evidently been continued into the Tertiary epochs that are represented by the Wasatch, Green River, and Bridger Groups, they might, doubtless, have continued their existence through those epochs as well as through the Laramie period but for the irruption of the mammalian hordes to which they probably soon succumbed in an unequal struggle for existence. According to the facts which I have here and elsewhere shown, we have in the strata of the Western Territories an unbroken record from the earlier Mesozoic far into Tertiary time, and consequently no complete line or plane of demarkation between them exists. Therefore the designation of any precise boundary between the Cretaceous and Tertiary of that region must be a matter of conventional convenience rather than of natural requirement.

ART. XXXVII.-SYNONYMATIC LIST OF THE AMERICAN SCIURI, OR ARBOREAL SQUIRRELS.

BY J. A. ALLEN.

Since the publication last year of my revision of the American Sciuri,* the "Neotropical" species of the group have been ably reviewed by Mr. E. R. Alston,† under unusually favorable circumstances. With his accustomed thoroughness, he has taken the trouble to seek out the types, so far as they are extant or accessible in several of the principal museums of Europe, of most of the species of former authors, and has thus been able to determine the character of many species so inadequately described, that in no other way could their proper allocation be satisfactorily determined. His careful elucidation of this obscure and perplexing group has not only placed his fellow-workers in the same field under lasting obligations to him, but must mark an era in the history of the subject. Of the fifty-nine nominal species of this group described by different authors, he informs us that he has examined the types of no less than forty-one! With the rich material of the British Museum at his command, he has been able to tell us exactly what the late Dr. Gray had for the basis of his nineteen "new species", described in a single paper in 1867, some of them so vaguely or inaccurately that the descriptions are sometimes misleading, and often inadequate indices of what he actually had before him. Mr. Alston has also been able to allocate the species described previously by the same author, and by Richardson, Bennett, Ogilby, and other British writers. In the Paris Museum, he found still extant the types of most of the species described many years since by Is. Geoffroy, Lesson, F. Cuvier, and Pucheran, and in the Berlin Museum types of the species described by Dr. Peters; so that the only important ones not seen by him are those of Brandt, Wagner, and Natterer. To assist him in collating my own work, I had the pleasure of sending him examples of the greater part of the species recognized by me in my recent monograph of the American Sciurida. As I had not access to the types of the species described by foreign authors, I made, in some instances, my allocations of synonymy with doubt, and, in other cases, only provisionally, feeling conscious of the uncertainty with which refer* Coues and Allen's "Monographs of North American Rodentia", pp. 666-797, August, 1877.

"On the Squirrels of the Neotropical Region ", Proc. Zoöl. Soc. Lond. 1878, pp. 656-670, pl. xli. This highly important memoir gives excellent diagnoses of the species, with their synonymy in full, and a critical commentary on the species of previous authors.

ences to many of the species must necessarily, under the circumstances, be made. Although Mr. Alston has shown the incorrectness of some of my identifications, and the necessity of substituting, in two instances, names other than those I was led to adopt, I feel, on the whole, no small degree of satisfaction in the confirmation of so large a portion of my synonymic work by the trying ordeal to which it has been submitted; especially as Mr. Alston has done me the kindness to state, in several instances, that I was led into mistakes by descriptions that did not properly represent the objects described. The purpose of the present paper is to correct these errors, so far as they have been satisfactorily shown, and to present a nomenclature that fairly reflects the present state of the subject.

In my former revision of the Sciuri of Tropical America, I felt authorized in reducing fully four-fifths of the previously described species to synonyms, and stated it as my belief that I had still recognized too many rather than too few. Mr. Alston, with far more-and mainly historic-material at his command, has, in one or two instances, carried the reduction still further, but, on the other hand, has added one or two species unrepresented in the material I had before me. While I recognized ten species and two subspecies, he has raised the number of the former to twelve. The changes, so far as species are concerned, consist in his elevating one of my subspecies to full specific rank; in treating as a species a form I regarded as the young of another species; in uniting, in two instances, two of my species into one; and in restoring two species I treated as nominal. These changes, as well as those of nomenclature and synonymy, will be fully noted in the following pages.

For the purpose mainly of presenting a connected view of the Amer ican Sciuri, but partly to correct one or two errors of synonymy, I include the North American species in the subjoined enumeration, although I have no changes to make in the nomenclature adopted in "Monographs of North American Rodentia". In order to distinguish readily those that are represented in the North American fauna, I divide the species, as before, into two geographical series. Gray's species are assigned in accordance with Mr. Alston's determinations, based on an examination of the types, as are also those of Peters, Pucheran, Cuvier, Geoffroy, Bennett, and Richardson. Consequently the synonymatic tables here presented are substantially the same as Mr. Alston's.

A.-NORTH AMERICAN SPECIES.

L-SCIURUS HUDSONIUS, Pallas.

1.-Var. hudsonius.

Sciurus vulgaris, FORSTER, Phil. Trans. lxii, 1772, 378.

Sciurus vulgaris, e, hudsonicus, ERXLEBEN, Syst. Anim. 1777, 416.

Sciurus hudsonius, PALLAS, Nov. Spec. Glires, 1778, 376.

Sciurus carolinus, ORD, "Guthrie's Geogr. (2d Am. ed.) ii, 1815, 292."

Sciurus rubrolineatus, DESMAREST, Mam. ii, 1822, 333.