104 PROCEEDINGS OF THE ACADEMY OF [1887. 



but we doubt if this is the case in the sense Dr. Carpenter suggasts. 

 The two former are Camarata, and as such should be j^rovided with 

 more than one row of interradials, which they Avould not posse&s if 

 the inner plates were orals. 



Based upon palaeontological evidence, we think, we may rea- 

 sonably suggest that in the developmental history of Culicocrinus, 

 at the close of the Inadunata stage, the first row of interradials 

 opened out to connect the proximal arm plate with the calyx, and 

 that a second ring formed to take the place and functions of the 

 first, closed the center. 



Coecocrinus forms a connecting link between Culicocrinus and 

 Platycriniis. Probably it has one or more summit plates, and the 

 ambulacra disposed between the interradials. 



In Platycrinus, the inner interradials, which in Coecocrinus are 

 yet placed at a level with the dorsal cup, are considerably more 

 raised. In consequence thereof we find in this genus much larger 

 spaces between the interradials, centrally as well as laterally, and 

 hence better developed summit plates and larger and heavier cover- 

 ing pieces. Of the summit pieces probably the central plate 

 appeared at first this is indicated by the phylogeuy of the group 

 and the proximals appeared later, filling the vacancies, which 

 gradually had formed around the central jjlate. 



In this sketch we have not added anything that is not well 

 sustained by the phylogeny of this group, or is not in accordance 

 Avith the developmental history of the Palaeocrinoidea generally- 

 Throughout this order, when summit plates are exposed at all, they 

 occupy a comparatively small space around the peristome, and this 

 space increases in width in palaeontological times. In all Palaeo- 

 crinoids, so far as known, and we may add, in all Blastoids, the 

 peristomial area is formed by the calyx interradials, whether these 

 consist of one piece, as in the case of the Inadunata and Blastoidea, 

 or of two, three, or a dozen pieces, as in the Camarata ; and the summit 

 i:)lates, whether composed of a central plate only, or of proximals 

 also, rest against the upper margin of the interradials. In all Neo- 

 crinoidea, however, from the larva to the adult, the whole ventral 

 surface is covered by actinal structures , the small interradials which 

 were observed by Sir Wyville Thomson, disappear again soon after 

 their development, and never attain any such prominence in the 

 composition of the calyx as in the earlier Crinoids. This character, 

 which distinguishes the two groups so readily, would meet with most 



