90 THE CRINOIDEA CAMERATA OF NOETH AMERICA. 



terior side to a nearly central position. The description of the ventral plates 

 of this genus in the Ke vision, Part III., p. 37, was incorrect as to the small 

 pieces around the five summit plates, the appearance of which was produced 

 by cracks and not sutures. Specimens of better preservation show positively 

 that the ventral surface is occupied exclusively by the five large plates. 

 Pisocrinus, as shown by a specimen ofP.pikda in our possession from Dudley, 

 England, has a similar structure. Among the Camerata five large summit 

 plates are known to exist only in Coccocriniis and CuUcocrinus, the plates 

 of the former being equal (Plate III, Fig. 14), those of the latter slightly 

 unequal. 



As the large central plate, when it occurs, occupies approximately the 

 same position as the oral opening of recent Crinoids, it seemed plausible that 

 the orals, if present at all, should be looked for in the Camerata among the 

 plates of the proximal ring ; but on examining the structure, it was found 

 that only the four larger plates could be compared with the five orals of 

 recent forms ; so the question arose, what had become of the corresponding 

 fifth plate ? 



Wachsmuth, in 1877,*^ directed attention to the two plates of the same 

 ring adjoining the four larger ones, and suggested that these two plates 

 taken together were probably equivalent to one, being split into two by the 

 anus, and that the six plates represented morphologically but five. The 

 plates were not, however, regarded by him as the orals ; he thought the 

 whole ventral covering of the earlier Crinoids was structurally distinct from 

 the disk of recent ones. 



A similar view of the subject was taken by us in the following year.f 

 We assumed that the plates of the dorsal cup and those of the tegmen were 

 parallel structures ; that the central plate was represented in the dorsal cup 

 or abactinal side by the infrabasals, the six proximals by the basals, and that 

 other plates of the tegmen represented the radials and interradials. No 

 comparison was then made by us of these plates with the plates in the disk 

 of recent forms ; but this was done in 1881,:j: when we suggested that the six 

 proximals represented the orals. 



Dr. P. H. Carpenter, like ourselves, recognized in the Camerata a central 

 plate, for which he proposed the name " oro-central," and six proximals 



v^rided our observation, and held the plate to be homologous with the so-called central plate of the Camerata. 

 (Chall. Rep. on the Stalked Crinoids, p. 158). 



* Amer. Journ. Sci. and Arts (series 3), Vol. XIX. pp. 186-187. 



f Revision, Part I., p. 28. 



I Revision, Part II. p. 17 (Proceed. Acad. Nat. Sci. Phila., p. 191). 



