110 PROCEEDINGS OF THE ACADEMY OF [1887. 



formatiou. The radial dome plates, as a rule, disappear when the 

 ambulacra enter the surface, and this explains why they are absent 

 in Blastoids,* Steplianocrinus and the later Cyathocrinidae. 



We have already alluded to the fact that the proximals are fre- 

 quently unrepresented in the earlier groups, in which, as a rule, the 

 peristomial area is comparatively smaller than in later ones, and 

 closed only by a small central piece. Upon this 2:oint it is very in- 

 teresting that w^e have recently discovered the same thing in later 

 groups under somewhat different conditions. In two cases, the one 

 a species of Talarocrinus from the St. Louis group of Kentucky (Fig. 

 10), the other a Dichocrinus from the Kinder hook of Iowa, w^e found 

 the whole space usually occupied by central piece and proximals, 

 although as large as in any Platycriniis, filled completely by an enor- 

 mous, nodose central plate, with the covering pieces abutting against it. 

 Interposed between the ambulacra are a number of small interradial 

 plates, which barely touch the central piece. In these cases, accord- 

 ing to our interpretation, the increasing space of the peristomial area 

 was filled by lateral growth of the orals (central piece), instead of 

 by means of proximals. But according to Carpenter's generaliza- 

 tions (Challenger Report, p. 171), the insignificant interradiaLs next 

 to the central piece, and between the amliulacra, should be the repre- 

 sentatives of the orals or else his rules would encounter another se- 

 rious objection. 



Dr. Carpenter regards both Allagecrinus and Haplocrinus as rep- 

 resenting in a phylogenetic sense embryonic stages of the Palaeocri- 

 noidea. If this be true, he has failed to give a reasonable explana- 

 tion how the large plates covering almost the whole ventral side in 

 these low foi^ms, came to be placed in this group so as to occupy only 

 the relatively small space they do in Avhat he regards as higher de- 

 veloped forms. Etheridge and Carpenter undertook to prove it in 

 their paper, AnnaLs and Mag. Nat. Hist., Apr. 1881, p. 289, by im- 

 agining that, in the more mature specimens of Allagecrinus "the 

 orals were relatively carried inwards, away from the radials, and 

 separated from them by perisome ( just as they are in the Pentacri- 

 noid larva of Comatula) when the arms appear above the radials. 

 Whether the orals ever separated so as to open the mouth to the ex- 

 terior, and whether the ring of perisome forming the ventral disk 



* Etheridge and Carpenter figure, Blastoid Catalogue on PI. XVIII, Fig, 

 16, Elaeacrinus Verneui/i with radial dome plates; none of our specimens show 

 any traces of them. 



