290 THE CANADIAN NATURALIST. [Sept. 



pores in Codaster are not pores, but the small pits or sockets 

 which received the bases of the pinnulae. The rays therefore in 

 this genus are not " pseudambulacral fields" in the sense in which 

 that term is used in descriptions of species of Pentremites, but 

 simply recumbent arms, identical in structure with those of the 

 cystidean genera Glyptocystites, Callocystites, Apiocystites, and 

 others. They lie upon the surface of the plates which constitute 

 the shell of the animals — not imbedded into them, as m Pentre- 

 mites. The large lateral aperture is both mouth and vent, and 

 the central opening, heretofore called the mouth, is the ambu- 

 lacral, or, more properly, the ovarian orifice. As, therefore, 

 Codaster has the arms of Apiocystites, the hydrospires of Pleuro- 

 cystites, and the confluent mouth and vent, common to all Cysti- 

 deans, I propose to remove it from the Blastoidea, and place it in 

 the order Cystidea. 



4. On the genus Pentremites. 



In Pentremites the hydrospire is an elongated, internal sack, 

 one side of which is attached to the inside of the shell, while the 

 side opposite, or toward the central axis of the visceral cavity, is 

 more or less deeply folded longitudinally. There are two of 

 these to each ambulacrum, attached along the two lines of pores. 

 There appears to be a fissure extending nearly the whole length, 

 in the direction of the dotted line /. One edge of this fissure is 

 attached to the lancet plate along one side of the line of pores ; 

 the other to the shell, on the other side of the row. The pores 

 all enter the hydrospire through this fissure. There are ten 

 hydrospires, connected together in pairs, each pair communicating 

 with the exterior through a single spiracle. The arrangement of 

 the folds varies according to the species. In P. Godoni there 

 are five folds, the outer sides of which are close up to the inner 

 side of the lancet plate, fig. 13. In a specimen of P. ohesus 

 Lyon, nearly two inches in diameter at the mid-height, the 

 hydrospires extend inward about three lines, the main body being 

 about one line from the lancet plate. There are five folds, each 

 two lines deep ; and thus, if the thin shelly membrane, which 

 constitutes the wall of the hydrospire, were spread out, it would 

 have a width of 22 lines, — and the ten together would form a 

 riband, about 18 inches in length, and nearly two inches wide. 

 The object of the folding is, of course, to confine this large 



