INVERTEBRATES. 181 



as in Dendrocrinus and some other allied genera. In P. missouriensis, Shumard, 

 {P. hngidactylus, Shumard, Missouri Report, pi. B, fig. 5), as well as several 

 other species we have examined, including some of the Scaphiocrinus group, 

 numerous slit-like pores are seen passing through the vertical zigzag sutures, 

 between alternate ranges of these small pieces, so as to present much the appear- 

 ance of the ambulacra of the Echinoids. excepting that these pores are not 

 round, and do not pierce the pieces themselves, but pass through the sutures. 



As already remarked, in connection with the description of that genus, the 

 group under consideration is in some respects closely similar to Cyathocrimis, 

 but differs remarkably in the size and structure of the superior parts of the 

 body, as well as in some less important points of structure in the body below 

 the arms. For all these differences see remarks on page 176. In the arrange- 

 ment of the pieces of the body below the arms, it agrees almost exactly with 

 llydrcionocrinus, deKoninck, though it differs completely above, in its arms 

 being free, instead of united laterally so as to form a kind of cylindrical tube. 



It is much more nearly allied to Zeacrinus, of Troost, from which it differs 

 mainly in its larger and more protuberant base, more numerous and narrower 

 radial pieces, as well as in the different form and proportions of its subradial 

 and anal pieces. As Zeacrimis, however, in some of its species, approaches 

 very nearly certain forms of Scaphiocrinus* which seems not generically distinct 

 from Poteriocrinus, it may be found that both Scaphiocrinus and Zeacrimis should 

 be ranged as subgenera under Poteriocrinus. 



If the type of Graphiocrinus, deKoninck and Lehon, should be found, as 

 suggested by Mr. Lyon and Dr. Shumard, to have a series of five minute basal 

 pieces within the range regarded as such by de Koninck and Lehon, which is 

 highly probable, then it could not be distinguished, even subgenerically, from 

 the typical species of Scaphiocrinus, in which case, the older name Graphiocri- 

 nus would replace 'Scaphiocrinus. 



The groups Scaphiocrinus, Hall, and Coeliocrinus, White, as originally intended 

 by their authors, appear to be sections or subgenera under Poteriocrinus. The 

 first, Scaphiocrinus, as represented by the typical species, S. simplex, differs 

 from Poteriocrinus proper, and approaches Zeacrinus, in having its base much 

 smaller and less protuberant, and in being provided with but two primary radial 

 pieces to each ray, while it also has but a single anal piece below the summit of 

 the first radial pieces. Such are the gradations presented by other species, 

 however, that Prof. Hall and others, who have recognized the group, have 

 found it necessary to include species with a larger protuberant base, and with 

 two anal pieces included in the wall of the cup below the top of the first radials. 



* It is acknowledged (Iowa Report, p. 682) that Zeacrinus intermedins, Hall, might 

 with almost equal propriety be referred to either Zeacrinus or Scaphiocrinus . 



