DESCRIPTIONS OF THE SPECIES. 'J 1 I 



Granatocrinus, we found that they belonged to two well-defined morphological groups. 



To the first of these, having for its type Pentremites Norwoodi, ( >. >.v s., we have else- 

 where proposed to restrict the name Granatocrinus ; while to the other, typified by 

 Pentremites Sayi, Shumard, we have applied the name Schizoblastus. 

 The form and general appearance of the calyx presents a remarkable uniformity of 



type throughout the species which we have retained under the name of Granatocrinus. 

 Two broad divisions may be traced — the first after the type of G. Norwoodi, the second 

 after that of G. ellipticus, G. B. Sby., sp. The form of the body is Bubglobose in 

 the first division, which includes all the British species except G. ellipticus and G. 

 McCoyi, These types, which are elongately elliptical (PI. VIII. figs. 10, 1 ( J; PI. X. 

 fig. 5), form a division by themselves, no other species resembling them in shape. The 

 summit is generally more or less flattened (PI. II. figs. 32, 33 ; PI. IX. figs. G, 11 ; 

 PI. X. fig. 9), or even at times a little depressed (PI. X. figs. 7, 16). The base is 

 usually small, and flattened or concave. The amount of the concavity varies with the 

 species, the basal plates never being visible in a side view. In G. Norwoodi the basal 

 concavity is narrow and deep (PI. III. fig. 16), in G. orbicularis broad and shallow 

 (PI. IX. fig. 13), the same in G. ellipticus (PL VIII. fig. IS), rather deeper in G. Der- 

 biensis (PL IX. figs. 3,4), and broad and almost flat in G. campanulatus, in fact 

 broader in this species than in any other (PL VIII. fig. 13). 



The relative proportions of the radial and deltoid plates vary considerably : some 

 species have large radials and small deltoids (PL II. figs. 32-35; PL VII. figs. 5, G) ; 

 others small radials and large deltoids (PL IX. figs. 1-6). Generic subdivision has 

 before now been attempted, according to the relative sizes of these plates ; but, as 

 Messrs. Meek and "Worthen have very justly observed, speaking of the deltoids in par- 

 ticular, " There are so many gradations in this character, however, that it does not 

 seem to be possible to make it a means of separating the species into two well-defined 

 sections " l . G. Norwoodi possesses very large radial plates, extending from the edge 

 of the hollow base almost to the very apex of the calyx, and correspondingly small 

 deltoids (PL VII. figs. 10, 12). On the other hand, in the British species, although 

 the radials maintain their general superiority of size in all but one form (PL IX. 

 figs. 1-4), the deltoids are longer than in the American type. A gradation, however, 

 is traceable from G. McCoyi (PL X. fig. 5), in which they are relatively smallest, to 

 G. orbicularis (PL IX. figs. 11, 12), which possesses the largest deltoid plates, with the 

 exception of G. Derbiensis (PL IX. figs. 1-4). In the latter species the deltoids attain 

 an inordinately large size, the radial plates being only just sufficiently high to enclose 

 within their sinuses the distal ends of the long ambulacra. In all the species, whether 

 the deltoids are large or small, the spiracles open on the mammillary projections at 

 the apices of these plates; but in G. campanulatus they are bounded by nodular 

 elevations of the plates (PL IX. fig. 8). Dr. C. A. AVhite has shown that the anal 



1 Iieport Ueol. Survey Illinois, lbUO, vol. ii. p. i!75. 



2i 



