INVERTEBRATES. 269 



Generic formula-. — 



Basal pieces, -3 — small, sometimes nearly hidden by the column. 

 Subradials, 5 — one larger than the others, and of different form. 

 Radials, 3 to 1 -{- 5, or varying sometimes in the different rays of the same in- 

 dividual. 

 Interradials, to 30, or more. 

 Anal plates, to 30 or 40, 

 Interaxillary pieces, to 10 (or more)-i}-5. 

 Arms variously divided. 



Basal pieces small, and often presenting much the appear- 

 ance of a thickened terminal piece of the column divided into 

 three parts. Subradials larger, and the one on the anal side 

 generally larger than either of the other four. Primary radial 

 pieces all alternating with the subradials — usually four to each 

 ray, but sometimes three, five, six or seven, the number being, 

 in some instances, different even in the different ray of the same 

 individual. All the primary radial pieces, as well as those of 

 the divisions of the rays, often with a very small supplementary 

 semicircular piece occupying a corresponding excavation at the 

 middle of the upper outer margin of each, and usually anchy- 

 losed to the lower margin of the succeeding piece above. Inter- 

 radial and anal pieces varying, in the typical species, from to 

 1, 2 or 3 to each space, and in the Forbesiocrinus section, from 

 about 7 to 10, 12, 15, etc., on up to 30 or more, to each space. 

 Divisions of the rays in the latter group sometimes soldered 

 together by small, intermediate pieces, so as to form part of the 

 walls of the body, and assume the character of secondary or 

 even Tertiary radials, before giving off the free arms. Vault 

 apparently only covered by a soft integument. Opening of the 

 summit unknown. Column round, generally large near the 

 base, where it is always composed of numerous, very thin 

 pieces. Surface nearly smooth or granular, and always without 

 radiating costse. 



As here defined, this group is made to include species differing greatly in 

 the number of anal and interradial pieces; but as we have elsewhere shown 

 (Proceed. Acad. Nat. 'JSci., Pldlad., Aug., 1865, p. 138), they agree so exactly 



