213 PROCEEDINGS OF THE ACADEMY OF [1885. 



Platycrinus. They form a ridge of strong tuberculous plates. 

 and are almost as prominenl as the apical or summit plates of 

 this species, while the in terradials, from the first to the last, are 



scarcely convex. The same structure is also found in Slegano- 

 crinus concinnus ( PL 8, fig. 4). In Carpocrinus ornatus, however. 

 the alternate plates retain, more or less, the character of other 

 perisomic pieces. 



Wherever covering plates in the Camarata are exposed, they 

 are invariably placed on a level with the in terradials, not upon 

 their surface, and the ambulacra are essentially in the same con- 

 dition as those of the A el inocrinidic, only the interradials do not 

 close over them, bul are pushed aside. The case, however, is very 

 different in the higher form of Gyathocrinus, in which not the 

 covering plates alone, but the whole tubular skeleton and the 

 entire disk is exposed. 



The discovery of anambulacral plates upon the surface of the 

 interradials is morphologically of the utmost importance, as 

 throwing light upon the phylogenetic as well as the ontogenetic 

 development of the older Crinoids and their relation to the 

 Neocrinoidea. II' a resorption of these interradial plates, as we 

 believe, took place in the Poteriocrinidse, then the dividing line 

 between the older and later Crinoids becomes so narrow, that it 

 is difficult to decide where the one terminates and the other 

 begins. A resorption of the interradial plates in palaeontological 

 times is in accordance with the embryological development of 

 recent Crinoids. Carpenter is inclined to believe (Chall. Rep., 

 p. 40), that the interradial plates, which SirWyville Thomson 

 (Philos. Trans., 1865, p. 540) observed in the early larval stages 

 of Antedon rosacea, and which he takes to be primary inter- 

 radials, " eventually undergo resorption like the orals and the 

 anal plate." 



I)i the Neocrinoidea, with the exception of Thaumatovrimi*. 

 Guettardicrihus, and one or two species of Apiocrinns, the inter- 

 radials are represented by indistinct plates, and are only tem- 

 porarily developed. In the Pahcocrinoidea, however, the inter- 

 radials are permanent, and in some groups so extravagantly 

 developed thai they constitute the greater part of the calyx. It 

 is very remarkable thai we find the most profuse development of 

 interradials among Silurian genera, which tends to prove thai a 

 largely developed interradial system represents a lower grade 



