1879.] NATURAL SCIENCES OP PHILADELPHIA. 305 



between this genus and Poleriocrinus. The truncated posterior 

 basal, the number and disposition of the anal plates, and the 

 regular alternate arrangement of basals and radials are also ex- 

 cellent characters by which the two genera may be easily distin- 

 guished ; but not the construction of the vault, nor the presence 

 or absence of a separate buccal aperture, as Austin and De Koninck 

 suggested; for both genera have a similar low vault with a single 

 aperture, and in both types the ventral sac is lateral, strong, and 

 upright, instead of extending from the entire summit like an 

 enormous proboscis, as it has been described by several authors. 

 De Koninck and Lehon give the number of anal plates at two to 

 six, but there is really but one plate that can be regarded as such, 

 all succeeding ones forming a part of the ventral sac. 



Hall, in order to admit into Gyathocrinus such types as were 

 afterwards separated under Barycrinus, mentions in the Iowa Re- 

 port that there sometimes occurs in the anal area a rather small 

 intercalated plate; but this, though having a similar position, 

 cannot be considered identical with the lower anal plate of Poterio- 

 crinus. Barycrinus has been very generally accepted as a genus, 

 and as the plate in question has only been observed in species of 

 that type, it need no longer be considered in this connection. 



Angelin, in the Iconographia Crinoideorum, p. 22, mentions the 

 presence in Gyathocrinus of small pinnulse. We have already 

 noticed this point in the introductory remarks, and think we have 

 proved that the two alternate rows of plates, there called pinnulse, 

 are merely plates which cover the ambulacral groove in the arms, 

 and though they are, in our opinion, the homologues of the pin- 

 nulse, they are too rudimentary to be ranked as such. The cover- 

 ing in Gyathocrinus longimanus Angl., from the Silurian, con- 

 sists of two rows of five successive plates each, one row being 

 given off from the right, the other from the left side of the furrow 

 and perfectly covering it. In Gyathocr. Iowensis 0. & Shum., from 

 the Subcarboniferous, there have been observed only two successive 

 plates arranged in the same manner. If it could be shown that 

 this structure were constant in all Silurian species of Gyatho- 

 crinus, we should feel disposed to separate them from their sub- 

 carboniferous representatives, at least subgenerically, since in the 

 former the arm-joints are comparatively shorter, the ventral tube 

 stronger, and there is besides a peculiar difference in the general 

 habitus of the two which is not easily expressed. 



