254 PROCEEDINGS OF THE ACADEMY OP [1885. 



The phylogenetic evidence indicates clearly that the inter radial 



element take- a most prominent part in the composition of the 

 Palaeocrinoidea, and we hope we have proved that these plates 

 were much more extravagantly developed in their earlier type-;. 

 In Silurian genera they extended over the whole peristome, or 

 the greater part of it. Gradually the summit plates made their 

 appearance, evidently pushed out from beneath, afterwards the 

 covering pieces of the ambulacra, and at last also the an am- 

 bulacra! plates. Even in the Cyathocrinidae, in which the ven- 

 tral structure attained a higher form than in any other group, 

 with the exception, perhaps, of the Poteriocrinidse and Encrinidae, 

 interradials are not only present, but they occupy the greater 

 portion of the ventral side, and even in those genera in which, 

 perhaps, they were resorbed before the Crinoid reached maturity, 

 they had been previously well developed. Under the weight of 

 this evidence, is it probable that Haplocrinus and AUagecrinus, 

 which are said to be " permanently in the condition of a very 

 early larva "' (Chall. Rep., p. 157), alone among all Palaeocri- 

 noidea, should have no interradial plates, and that the plates 

 which occupy their position in these two genera are "oral 

 plates ?" We, at least, wish to be excused if we doubt it. Upon 

 palaeontological grounds we expect to find in the j^ounger stages 

 of the Palaeocrinoid the oral S} r stem feebly, the interradial sj'stem 

 extravagantly developed, while, according to Carpenter's inter- 

 pretation of the plates, in the Palaeocrinoid larva, the entire 

 ventral surface from the radials up would be oral, i. e, actinal. 



From an embryological standpoint also, Carpenter's interpre- 

 tation meets with very serious objections. If Haplocrinus rep- 

 resents, as he asserts, a very early stage in Crinoid ontogeny, 

 before the opening of the tentacular vestibule to the exterior, we 

 should like to know how the central piece, the so-called orocen- 

 tral of Carpenter, made its appearance in the Palaeocrinoid. It 

 is not very probable that this plate was present in the earl}- 

 larva, or it would certainly be represented in the larva of the 

 Neocrinoid at the time the oral pyramid was closed. Carpenter 

 claims that it was even unrepresented in Allagecririus, and that 

 the oral pole was closed only by oral plates. This would suggest 

 that it was introduced either by means of a partial resorption of 

 I he " oral " pyramid, or ly the opening of its plates. The former 



is exceedingly doubtful, while the latter is clearly not the case in 



