INVERTEBRATES. 275 



through them, so that each pair appears externally, as a single opening ( G. melo 

 i;nd G. Norwoodi), though they divide into two distinct canals before passing 

 entirely through the plates.* The typical forms of this genus also have the 

 interradial pieces proportionally much larger than in the true Pentremites, 

 though this is not a constant character. 



In the possession of numerous, extremely slender, thread-like, simple arms, 

 arranged along the pseudo-ambulacral areas, this type also agrees (as might 

 have been inferred from analogy) with the true Pentremites, as we know from 

 the examination of a beautiful specimen belonging to Mr. Wachsmuth. This 

 specimen seems to be related to G. Nbrwoodi, as near as can be determined, 

 and shows at least thirty (there are probably more) of these delicate, simple 

 arms, arising from each pseudo-ambulacral area, and extending up so that the 

 lower ones must be quite twice as long as the body. They are all composed of 

 equal joints, about as long as wide. So far as we know, this is the only exam- 

 ple of a specimen of this type showing the arms, yet found. 



This group is even more nearly similar in general appearance, to Nucleocrinus, 

 Conrad = (^Elseacrinus, Eoemer), from which it differs in important points of 

 structure. For instance, in Nucleocrinus, we find the area between two of the 

 pseudo-ambulacra on the anal side wider, and often more prominent above 

 than the other interambulacral spaces, and occupied by three large, elongated 

 pieces, the middle one of which, the anal piece, is lanceolate in form, and with 

 the two interradials fills all the large anal area down to the base. On compar- 

 ing this structure with Granotocrinus, the latter is found to differ in having no 

 such three elongated pieces on the anal side ; but only a single anal piece differ- 

 ing little from the interradials. So marked a difference as this, in the structure 

 of such simple types, is of course incompatable with generic identity. 



As now understood, this genus includes species differing materially in the 

 comparative size of the interradial pieces, the typical species having those 

 pieces very large ; while in another section of the genus, represented by such 

 forms as G. melo and G. Nbrwoodi, they are as small as in Pentremites. 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 sec- 

 tions. 



This genus is, so far as known, confined to the Subearboniferous series. 



*In worn x>r weathered specimens of G. melo and G. Norwoodi, these little interra- 

 dial pieces, have been so much eroded as to expose each pair of these openings entire- 

 ly distinct, when they were really united and appeared as a single opening externally, 

 before wearing. 



