connected with the ambulacral system, and I am not aware of the existence of the true respiratory double pores except in connection with branches from the water-vessels. Without a special vessel, the arrangement would be useless. It is very likely that the arms, which are tolerably well developed in the true Sphæronites with single calycine pores, may have been rudimentary in the Diploporitidæ. The arms of Crinoids are developed round the mouth, in the position of the branchial tufts of the Holothuriadæ, with which group the Crinoids, from the very earliest stages of their embryonic development, present some very remarkable analogies. The radial plates in the Crinoids. are not developed, as has been imagined, from the apical pole ; but when the radial canals begin to sprout round the mouth, the first radial plates are intercalated, as it were, from above, between the already formed basal and first interambulacral series, like brackets for the support of the growing arms. Echinocystites seems to me to be one of the transition forms in the steady main line of advance of the class towards its apparently most condensed and characteristic development in Echinus. Sphæronites has lost its arms and all that remained to it of a stem, and the Diploporitidæ have got their pores ranged into line to perform the new duty of locomotion. Parting with Sphæronites, the main body finally takes leave of the Crinoids, which thence diverge to follow, to the death apparently, through the Palæozoic, Triassic, Jurassic, and all the countless lost ages, their own excentric ordinal idea of frittering away their whole substance in a mass of tentacles and plumes. Echinocystites is not the sole representative of its group. Agelacrinites, which seems to have made its appearance before it, and which reappears among the Blastoids in the mountain limestone, is closely allied, although our specimens of this genus are as yet scarcely sufficiently numerous and complete to furnish the details requisite to a full analysis; and the Echinoids of these early days seem to have had their Clypeastroid relatives in Mr Salter's genus Palæodiscus. It is unfortunate that our materials for the illustration of this beautiful genus are still very imperfect. All the specimens in the College Museum in Belfast, and, I believe, all those in the Ludlow Museum, are impressions, in some form or other, of the oral surface. One of my specimens (Plate IV. fig. 6) is a mould of the interior of the oral surface, and another (Plate IV. fig. 7) is a cast of the same aspect, this latter, however, fortunately preserves some imperfect impressions of the interior of the apical perisom. A third is an impression of the outside of the oral surface; and a fourth is a mould of the last, which has carried with it and preserved the close felting of minute spines. Palæodiscus (Salter). Plate IV. figs. 6-8. Palæodiscus ferox (Salter), the only known species. The body is very much compressed; the oral surface is discoid, slightly arched, about an inch and a half in diameter; the mouth is central, small, furnished with a much depressed cone of five pairs of strong, double, wedge-shaped jaws, ending in double, striated chisel-like teeth; the cone is within the perisom, as the ambulacral plates pass over it, close up to the margin of the mouth. The ambulacra are five in number, slightly petaloid in form, contracted towards the mouth, and again narrowing at the margin. They are continued over the apical surface to the apical pole, but apparently not in the same petaloid form. One of my specimens shows one of the ambulacra doubling back as a linear groove, with a row of single orifices, probably for the passage of locomotive suckers on either side. The ambulacra of the oral surface are composed of a double row of linear ambulacral plates, closely ranged, and in my specimens, doubtless owing to their state of preservation, presenting no visible apertures, either through or between them. An," avenue," following the contour of the ambulacra, runs along the centre, and a central ridge indicates the suture of the ambulacral plates. A row of linear adambulacral plates fringes either side of each ambulacrum, which it nearly equals in width. These plates seem equal in number to, and to alternate with, the ambulacral pore-plates. The rest of the interambulacral space is filled up by a close-fitting pavement of smooth or finely granulated plates, irregular in number and form. These plates, as well as the plates of the ambulacra, are densely covered with a velvety pile of smooth fine spines. The apical integument seems to have been supported by irre |