304 CYSTOIDEA, BLASTOIDEA, AND HOLOTHUROIDEA. 



The genus Pentremites 1 itself (fig. 179, a and b) is much 

 the most important, but as its structure has been taken as 

 typical of that of the entire order, nothing further need be 

 said on this head here. The species of the genus range from 

 the Upper Silurian (P. Rheinwardtii, Troost) to the Carbon- 

 iferous, but they attain their maximum in the latter period, 

 and are not known in any higher deposits. Very nearly 

 allied to Pentremites is the genus Nudeocrinus ( = Eleacrinus, 

 Eoemer), which differs from the former chiefly in the fact 

 that one of the deltoid plates is split into two by the inter- 

 calation of a supplementary anal plate, and by the compara- 

 tively small size of the forked " radials " and the correspond- 



Fig. 179. Morphology of Blastoidea. a, Pentremites pyriformis, viewed sideways, showing 

 a portion of the column ; b, Summit of the calyx of Pentremites cervimis, showing the pseud- 

 ambulacral areas "and the apical apertures; c, Side view of Granatocrinus melonoides ; d. 

 Summit of Granatocrinus neglectus. (Figs, a and 6 are of the natural size ; c and d are slightly 

 enlarged.) (After Hall, and Meek and Worthen.) 



ingly increased length of the " deltoids." The genus is char- 

 acteristically Devonian, but extends into the Carboniferous. 

 Granatocrinus, again (fig. 179, c and d), is intermediate as 

 regards the relative size of its " radials " and " deltoids " 

 between Pentremites and Nudeocrinus, and it is peculiar in 

 having the pseud-ambulacra very narrow, and so greatly 

 elongated as almost to divide each radial plate into two 

 pieces. The genus is confined to the Carboniferous rocks. 

 Lastly, we may provisionally place in the Blastoidea the 

 extraordinary genus Astrocrinites, of the Carboniferous rocks, 

 which agrees with Pentremites in general form and structure, 



1 The generic name has been often, but unnecessarily, changed, upon philo- 

 logical grounds, to Pentatremitcs or Pcntatrematites. 



