102 



toward the central axis of the visceral cavity, is more or less deeply fold- 

 ed 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- 

 Fig. 60 



9 



Fig. 60. — Diagrams of one pair of the hydrospires of a Pentremite^—a. the inner side ; 6, the 

 outer, or side attached to the shell ;/, the fissure. 61. SeAion across an ambula- 

 crum of a specimen of P. Croc/onCenlarged 3 diameters. — I, lancet plate ; g, ambula- 

 cral groove ; p, pores leading into the hydrospires ; h, h, the two hydrospires, in 

 transverse section. 62. Ideal figures of a transverse section through an entire spe- 

 cimen showing the ten hydrospires, — I, one of the five lancet plates ; pj p, pores? 

 r. r, the two branches of one of the radial plates. 63. Summit of P. conoideus^ — a, 

 anterior side ; g, ambulacral grooves (copied from Dr. Shumard, but with the ova- 

 rian pores added). 



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.Godoui there are five folds, the outer sides of which 

 are close up to the inner side of the lancet plate, fig. 61. In a specimen 

 of P. obesus 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 ; 



