258 PROCEEDINGS OF THE ACADEMY OF [1885. 



interratlial plates were orals. In the latter genus, the orals rest 

 against the radials, and the ambulacra are exposed only along the 

 arms. In Coccocrinus, however, the so-called " orals " abut with 

 their outer ends against the interradials, and the clefts from the 

 " orals," in place of entering the arms, are continued between the 

 interradial plates. 



In Part II, p. 58, we asserted that the clefts along both plates 

 were probably filled in the animal by alternate (covering) pieces, 

 and the summit openings by dome plates ; although regarding at 

 that time the inner circlet of interradials as oral pieces. We 

 admit that Carpenter is right in asserting that the existence of 

 covering plates between the orals is contrary to the structure 

 of recent Crinoids, and at variance with the nature of oral 

 plates generally ; but considering, as we do now, that the inner 

 as well as the outer plates are interradials, this objection loses 

 its force, since covering plates are found between interradials in 

 most of the Platycrinidse. Yet the case of Coccocrinus is some- 

 what different from that of an ordinary Platycrinoid, which 

 together with covering pieces has well-developed summit plates, 

 of which no trace has been found in any of the specimens of 

 Coccocrinus. Carpenter thinks that in Coccocrinus the central 

 piece was unrepresented, that its five inner interradials are ho- 

 mologous with the six proximals of Platycrinus, and that the 

 tentacular vestibule with the mouth at the bottom was exposed 

 to view. This interpretation is a natural consequence of his 

 oral theory, and shows still more forcibly the difficulties of 

 his position. Not only has he to admit a homology of five 

 plates to six, but that in a Silurian genus mouth and food 

 grooves were not covered. This assumption, which represents 

 an enormous advance in the development of the group, not 

 attained by any other Palseocrinoid, is alone sufficient to over- 

 throw his whole theory, and this the more when applied to a 

 genus which decidedly represents a low stage among these 

 Crinoids. What is left to make Coccocrinus a Palaeocrinoid ? 

 Even the asymmetry, which, according to Carpenter, is one of 

 the best characters for separating the older and later Crinoids, 

 is rather problematical, as it has no special anal plate. 



Admitting that the inner plates in Coccocrinus are secondary 

 interradials and not proximals, we have to account for thr ab- 

 sence of i hese plates in this case. That the summil plates, which 



