1885.] NATURAL SCIENCES OF PHILADELPHIA. 305 



and sometimes connected laterally by a membranous substance. 

 The ventral surface, so far as known, is composed of interradial 

 plates ; it forms a pliable vault, which extends to the free rays, 

 and probably covers not only the disk, but also the summit 

 plates. The Crotalocrinidre have no anambulacral pieces, but 

 possess hydrospires within the calyx. 



The Inadunata are subdivided into Larviformia and Fistulata. 

 They include all Palaeocrinoidea in which the arms are free from 

 the first radials. Their calyx is comparatively small ; composed 

 exclusively of basals, frequently underbasals, five radials, five 

 interradials, and one or two azygous plates. The proximal ring 

 of plates, whether basal or underbasal, is composed of five, or 

 less frequentby, three plates. The radials are laterally connected 

 except at the posterior side, where they are separated by an 

 azygous and anal plate, if these have not been resorbed. The 

 presence of the azygous plate gives to the calyx a ver} 7 irregular 

 outline. The interradials are located ventrally; they rest against 

 the upper ends of two adjoining radials, and join along their 

 lateral margins. 



The ventral covering of the Larviformia consists of com- 

 paratively few pieces, among which generally the combined muscle 

 plates form a conspicuous part. The central space is covered 

 either exclusively by interradials, or these enclose an oral plate, 

 which in some of the higher forms is surrounded by proximals. 

 The disk is subtegminal in place of being extended into a lateral 

 sac. The anal opening either penetrates the interradials, or is 

 placed intermediate between two radials or their appendages. 

 Respiration took place b} r pores along the arm furrows, which 

 probably communicated with hydrospires. 



In the higher organized Fistulata the perisome is partly or 

 wholly exposed, the interradial plates either cover the perisome, 

 or this parti}' covers them. In the latter case the summit plates 

 may be resorbed, in the former they are largely represented ; but 

 in either case portions of the disk penetrate the catyx posteriorly 

 b} T passing out through the anal opening. These portions form 

 either a balloon-shaped or a tubular sac, composed of well-defined 

 plates, closed at the end, but perforated over the surface by pores 

 along the suture lines ; the pores penetrating the lateral edges of 

 the plates. Respiration took place by means of the pores alono- 

 the perisome. 

 21 



