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Show 996 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, of any anchoring structure, but is in all respects adapted for free locomotion; the calycal cavity is large in proportion to the thickness of the arms, and is enclosed by thin flexible walls. Of these three genera, Saccocoma is the most specialized; as Otto Jaekel concludes in his detailed and interesting account, " The totality of organization and the mode of occurrence of the Sac-cocomida? indicate that they were pelagic animals, and that, as such, they not merely lived in swarms, but inhabited every peaceful basin of the Solenhofen sea in enormous numbers" (Zeitschr. deutsch. geol. Gesell. xliv. p. 689,1893). Marsupites also was perhaps pelagic; the plates of its skeleton are, as I intend to show in another paper, of very light construction and with flexible unions ; the specific gravity of the whole animal must have been light and perhaps still further lightened, as Jaekel suggests for Saccocoma, by " a slight development of gas within the body." Uintacrinus, wdth its large calyx, its thin flexible test, its extraordinarily long and movable arms, appears likewise to possess the characters of a pelagic organism; and so far as the argument from mode of occurrence is of any value in the case of Saccocoma, it is just as applicable in the case of Uintacrinus, or at all events U. socialis, which lived in similar swarms and is buried in a similar deposit. As for U. westfaliens, its gregariousness may be open to dispute, but it is to be noted that the one specimen known occurs in association with Marsupites. At any rate, Uintacrinus, Marsupites, and Saccocoma appear to have had much the same mode of life, and to have been subject to similar environment. Let m e repeat that the word group, as used in the preceding paragraph, is of physiological and not morphological significance. It implies identity of condition but not of ancestry. Take any one of these groups, and what could be more divergent than the forms therein included ? Thaumatocrinus is essentially so unlike Antedon that, had the two genera not chanced to be both furnished with a centrodorsal, not a soul could have been led to place them in a single family, or even, one would imagine, in a single order. In the second group, Agassizocrinus is a dicyclic Inadunate, apparently allied to Cromyocrinus; Eelriocrinus is a monocyclic Inadunate of obscure, but undoubtedly very different, affinities; Millericrinus pratti is but a single species of a well-known genus of Penta-crinida?, and is pseudo-monocyclic. So is it with our third group : Saccocoma has a cup of nothing but radials ; Marsupites has radials, basals, and infrabasals ; Uintacrinus has no infrabasals, but, in addition to its basals and radials, has brachials, interbrachials, interdistichals, pinnulars, and interpinnulars, all helping to compose its dorsal cup. Admitting the essential dissimilarity of the three forms in our third, or pelagic, group, w e see the sooner what are the secondary features due to environment, the necessary consequences of their line of evolution. They are the features in which these three dissimilar forms have come to resemble one another. The thinness of the test, the large size of the calycal cavity, the flexibility of |