284 PROCEEDINGS OF THE ACADEMY OF [1881. 



cinnus Lyon and Casseclay, which agrees with the latter in all 

 essential points, but differs in having an anal tube in place of an 

 opening directly through the vault. 



Generic Diagnosis. — General form of the body pyramidal, wider 

 than high; symmetry decidedly bilateral; plates without surface 

 ornamentation. The form of the apical side varies from a shallow 

 basin to an inverted cup, and hence from convex to deeply con- 

 cave ; the concavity sometimes involving the third primar}' and 

 even partly the secondar}^ radials. Dome surpassing the calyx in 

 height, composed of large nodose or tuberculous plates, which are 

 surrounded by smaller scarcely convex pieces. 



Basal disk in form of a hexagon with nearly straight sides, 

 composed of three equal plates, ver^^ small, frequently hidden 

 from view by the column, spread out horizontally, and forming a 

 small concavit3^ Primary radials 3X5; the first comparatively 

 small, hexagonal, the upper lateral sides shorter than the other 

 sides ; second radials quadrangular or nearly so, smaller than the 

 first, transversely arranged ; third pentagonal, wider than high, 

 larger than either of the others. The latter support upon each of 

 their upper sloping faces a wide but short secondary radial, which 

 is succeeded by still shorter plates which gradually interlock and 

 become regular arm plates. All plates above th3 third radials 

 project at nearh'' right angles to the vertical axis, their faces 

 directed laterally. This gives to the a'pical side, when viewed 

 from the column, a pentalobate outline, and the latter plates appear 

 as parts of the free rays. The plates, however, are wedge-shaped, 

 knife-like toward the inner side, and the ambulacral passages 

 within the body communicate directly with the arm furrows. In 

 species with tertiary radials, and consequently a larger number of 

 arms, the last bifurcation takes place upon the first secondary 

 radials, but generally only upon one side and in the posterior 

 rays. There seems to be one or two undescribed species in which 

 the postero-lateral rays have normally four arms. We have in our 

 possession a specimen from Canton, Ind., in which also the antero- 

 lateral rays have an additional arm. 



Arms robust, long, simple, gradually tapering and terminating 

 in a sharp point. They are constructed of two rows of pieces, which 

 are alternatel}^ arranged, very wide, exceedingly short and linear. 

 Arm furrows wide but shallow ; pinnules slender, thread-like, com- 

 posed of cylindrical joints covering the arm furrows like a roof. 



