116 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



processes; the genital pinnules are very rarely laterally expanded, and the first 

 pinnule differs from the succeeding, which it resembles in its general character, in 

 being greatly enlarged, w-ith large stout segments, or, more rarely, reduced in size; 

 in the few genera in which the latter condition obtains the cirri are enormously 

 elongated (figs. 4, p. 63, 18, p. 67, 46, 49, 53, 54, p. 81, 93, p. 153, 94, p. 155, 95, 

 p. 157, 96, 97, p. 159, 361-362, p. 295, and 363-368, p. 297). 



If the pinnules are neither prismatic nor provided with well-developed side 

 and covering plates, they should be examined to determine the proportionate length 

 of those in the mi(lcll(\ and distal part of the arm; if the middle pinnules arc notice- 

 ably longer than the distal, the cirri must be consulted; if these are short and stout 

 and composed of subequal squarish segments, the outer with two dorsal transverse 

 ridges (see fig. 353, p. 293), and if the first pinnule is longer and larger than the suc- 

 ceeding, the specimen belongs to the genus Oligomef rides; but if the cirri, while stout, 

 are perfectly smooth dorsally, and the first pinnule is more slender than the one 

 succeeding, the family Tropiometridte is indicated (see figs. 88, p. 145, and 356, 

 p. 293). 



If the distal pinnules are longer than the middle pinnules, the possession of a 

 large and prominent conical centrodorsal bearing cirrus sockets in regular well 

 separated colunms, each socket being surrounded ventrally (proximally) and 

 laterally by a high prominent more or less horseshoe-shaped rim, and of true basals 

 visible between the centrodorsal and the radials, as well as the entire absence of 

 pinnules from the proximal 10 or 11 brachials, denote the family Atelecrinidse 

 (figs. 123, p. 192, 124, 125, p. 193, 218, 223, p. 243, 227, p. 245). 



For the determination of the remaining families the arms offer perhaps the best 

 index; there may be 20 arms, arising from 10 radials, each post-radial series dividing 

 once; such a condition is only found in the Antedonidae in the genus Promacho- 

 crinus: there may lie 10 arms arising from 5 radials, each of the post-radial series 

 dividing once; or there may be more than 10 arms. 



If there are more than 10 arras the second division series (IIBr series) may 

 consist of either two or four ossicles, in the latter case the two outer elements being 

 always united by sjv^yg}'. 



If the IIBr series are 4(3+4) the specimen belongs to the Himerometridse 

 (fig. 85, p. 139); if these are 2, it may belong to the Stephanometrida?, 

 the Mariametridse, or the Colobometridse ; the species of Colobomctridie which 

 have more than 10 arms are very easily differentiated from the multibrachiate 

 representatives of other families bv their stout cirri which arc composed of sub- 

 equal segments, those in the outer part bearing paired dorsal spines (see figs. 

 87, p. 143, and 345, p. 289); in the Stephanometridse one or more of the proximal 

 pinnules is enlarged, greatly stiffened and spine-like, but composed of usually 

 less than 15 segments, most of which are elongated (see fig. 6, p. 63); the divi- 

 sion series also are rather widely separated, and each of their component ossicles 

 bears a ventrolateral process; in the Mariametridse the di\'ision series are usually, 

 though not always, dose together laterally, and may be laterally flattened; the 

 proximal pinnules, though sometimes more or less enlarged, are flagellate and are 

 composed of over 20 segments. 



