1879.] NATURAL SCIENCES OF PHILADELPHIA. 333 



in shape, subcorneal or subturbinate, sometimes bell, cup, or almost 

 saucer-shaped; arrangement of plates unsymmetrieal. 



Underbasals five, equal in form and size, forming a more or less 

 depressed pentagonal cup. Basals five, three of them hexagonal 

 or heptagonal, and of the same size, the two others those adjacent 

 to the anals slightly larger, and with one or two additional sides. 

 Radials wider than high, four of a similar pentagonal form, and 

 alternating regularly with the basals; the fifth slightly smaller, 

 elevated above the level of the others, less regular in form, resting 

 upon the truncated upper side of one basal and against two anal 

 pieces. Articulating surface very variable, the scar for the attach- 

 ment of the brachials occupying in the typical species scarcely 

 one-third of the upper edge, but in some species it extends across 

 the entire margin of the radials with a hinge-like apparatus for 

 articulation. Brachials one to two or more, their number the 

 same in four of the rays, the anterior having often a few additional 

 plates. In species with only one brachial it is generally long and 

 laterally constricted; in those with two brachials, the first is 

 quadrangular and short, and both combined are equivalent in form 

 and size to the single plate of the other type. 



Arms simple or branching; the anterior ray, frequently less 

 developed than the others, is sometimes simple throughout ; com- 

 posed of single joints, generally at least as high as wide, often 

 much higher, more or less wedge-shaped, and throwing off pinnulse 

 alternately. In species whose pinnules have a decidedly zigzag 

 arrangement the}' are remarkably strong. 



Anals three (rarely four) within the calyx, in two rows alter- 

 nately arranged; the lower one pentagonal, resting against the 

 sloping upper side of the posterior basal and the radial to the 

 right; the second supported by the upper truncated side of the 

 posterior basal, having on the left a radial, on the right the first 

 and third anal plates, the third being sometimes partly above the 

 level of the calyx. Succeeding plates hexagonal, decreasing in 

 size upwards, forming a part of the ventral sac. Ventral sac up- 

 right, strong, cylindrical, sometimes club-shaped, either extending 

 up to the top of the arms or more than half wa} T , frequently 

 crowned with a set of long spines. Plates of the sac hexagonal, 

 and pierced at the sutures with pores or fissures, which penetrate 

 the lateral margins of the plates. Anal opening rarely observed, 



