43^ 



PELMATOZOA. 



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crinus (Ordovician), and Homocrinus (Silurian and Devonian) are other 

 leading genera of this family. 



Family 17. Poteriocrinidce. — In this family the calyx is "dicyclic," 

 and is essentially similar to that of Cyathocrinus, consisting of five 

 underbasals, five basals, five primary radials, and from one to 

 five interradials (fig. 291). The arms are simple 

 or bifurcated, and carry long pinnules. The ven- 

 tral surface of the calyx is elevated and convex, 

 and a greatly developed anal tube or proboscis is 

 usually present (fig. 298). The genera of this 

 family appear to be principally, if not wholly, con- 

 fined to the Devonian and Carboniferous rocks, 

 by far the larger number of forms belonging to 

 the latter. 



The principal genus of this family is Poteriocrinus 

 itself (fig. 298), which is closely allied to Cyathocrinus, 

 from which it is distinguished principally by the more 

 elevated form of the ventral sac, the great develop- 

 ment of the anal tube, and the structure of the arms. 

 These latter organs may be simple or branched, but 

 in either case are provided with pinnules, while the 

 radials are truncated superiorly, where they articu- 

 late with the brachials. On the other hand, in 

 Cyathocrinus the arms are without pinnules, and 

 the first radials (fig. 308, r) articulate with the second 

 by means of horse-shoe-shaped facets. The species 

 of Poteriocrinus are mainly Carboniferous, but a few 

 forms are known from the Devonian rocks. Closely 

 allied genera are Zeacrinus (Carboniferous) and 

 Scaphiocrinus (Carboniferous and Devonian). 



Family 18. Astylocrinidce. — The calyx in this 

 family is free, the peduncle of the larva being 

 evanescent ; and it is composed of firmly united 

 massive plates (fig. 310). Typically, the calyx is 

 " dicyclic," but Edriocrinus, if rightly referred 

 here, has no underbasals. There are two cycles of 

 radials, the second of which is axillary, and the 

 arms are provided with long pinnules. Anal interradial plates are 

 developed, and the dorsal side of the calyx does not carry cirri. The 

 type-genus of this family is Agassizocrinus ( = Astylocrinus), which 

 is found in the Carboniferous Limestone of North America. The 

 genus Edriocrinus, from the Silurian rocks of North America, has 

 also been provisionally placed in this family by Wachsmuth and 

 Springer. 



In Agassizocrinus (figs. 310, 311) the calyx was attached in its early 

 condition by a larval peduncle, but the adult was free. The underbasals 



Fig. 309. — Side-view 

 of the calyx and arms 

 of Cyathocrinus longi- 

 manus, of the natural 

 size, Silurian. (After 

 Angelin — copied from 

 Zittel.) 



