996 MB. r. A. BATHEE OS TriNTACHiNTJs. [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, Saecocoma 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- 

 cocomidae 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). Marstqntes 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 Saecocoma, by " a slight development of gas within the body." 

 Uintacnnus, with its large calyx, its thin flexible test, its extra- 

 ordinarily long and movable arms, appears likewise to possess the 

 characters of a pelagic organism ; and so far as the argiunent from 

 mode of occurrence is of any value in the case of Saecocoma, it is 

 just as applicable in the case of Uinfacrinus, or at all events 

 U. socialis, which lived in similar swarms and is buried in a similar 

 deposit. As for U. westfalicus, 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, Vintacrinus, Mar- 

 sivpites, and Saecocoma appear to have had much the same mode of 

 life, and to have been subject to similar environment. 



Let me 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 coidd 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 dicycUc Inadunate, apparently 

 allied to CromyocririKs ; Eclriocrinus is a monocyclic Inadunate of 

 obscure, but uadoubtedly very different, afiinities ; Millericrinus 

 j)ratti is but a single species of a well-known genus of Penta- 

 crinidae, and is pseudo-monocycUc. So is it with our third group : 

 Saecocoma has a cup of nothing but radials ; Marsiqntes has 

 radials, basals, and infrabasals ; Uintacrinus has no infrabasals, 

 but, in addition to its basals and radials, has brachials, inter- 

 brachials, 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, we 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 



