248 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



or truncated hemispherical. The cirrus sockets are usually somewhat larger than 

 in the Thalassometridse, and are arranged in from one to three more or less irregu- 

 lar rows which may be alternating, but usually show more or less of an approach 

 to a columnar disposition, three columns to each radial area (instead of two as is 

 mostly the case among the Thalassometridse) , of which the median may be wanting 

 (veiy rarely the two lateral), leaving a broad bare space between the remaining 

 columns. The dorsal pole is broad and flat and more or less deeply sculptured, 

 but there is no other ornamentation, except in Crinometra (fig. 206, p. 239), where 

 the dorsal pole, the surface of the centrodorsal between the cirrus sockets, and its 

 inferior border are usually covered with fine spines or tubercles corresponding in 

 character with those on the radials. In the cases where definite radial areas are 

 discernable they are delimited merely by more or less broad areas bare of cirrus 

 sockets, these being in the most extreme cases only slightly more convex than the 

 remaining surface of the centrodorsal, and never rising into prominent ridges as in 

 the Thalassometridse. 



The generic differentiation shown in the centrodorsals of the Charitometridse is 

 a useful supplement to determination based upon other characters, though used 

 alone it is somewhat uncertain. Owing to the proportionately large size of the 

 centrodorsal (resulting from the comparatively small amount of that surface resorp- 

 tion which is carried to an extreme in the Thalassometridse) the central cavity is 

 relatively small. 



The large, usually highly spinous or tubercular, centrodorsals hi Crinometra, 

 as well as the more or less definite arrangement of the cirrus sockets upon them, 

 make the identification of the species of that genus comparatively easy; Pachy- 

 lometra and Glyptometra (fig. 100, p. 162) have very large and thick centrodorsals, 

 unornamented, the cirrus sockets arranged as in Crinometra (though showing a 

 tendency to drop out the central column in each radial area), about of the same 

 size, and about as numerous; these two genera can not be distinguished from each 

 other by then" centrodorsals; Pcecilometra, Charitometra, and CJilorometra (figs. 99, 

 p. 160, and 207, p. 239) have smaller centrodorsals which are proportionately higher 

 w"ith smaller polar areas, approaching a low truncated conical or hemispherical 

 shape. We have not as yet sufficient knowledge of the component species of these 

 three genera to determine positively whether or not the type of centrodorsal found in 

 each is characteristic, though in CJilorometra one of the species groups has the cirrus 

 sockets in a single column in the center of each radial area as a result of the sup- 

 pression of the two lateral columns. Strotometra (figs. 101, 102, p. 163) has a thinner 

 centrodorsal than any of the other genera, and it bears fewer cirri, these being in a 

 single marginal row. 



In the Antedonidse (figs. 103, p. 165, 104, p. 167, 105, p. 169, 106, p. 171, 107, 

 p. 173, 108, p. 174, 109, p. 175, 110, p. 176, 111, p. 177, 112, p. 179, 208-216, p. 241, 217, 

 219-222, 224-226, p. 243, and 228, p. 245), Atelecrinidse (figs. 123, p. 192, 124, 125, 

 p. 193, 218, 223, p. 243, 227, p. 245, and 414, p. 319), and Pentametrocrinidse (figs. 113, 

 114, p. 181, 119, p. 185, 120, p. 187, and 121, p. 189), which together form the suborder 

 Macrophreata, the centrodorsal is usually very large and deep, and the inner prox- 

 imal border is commonly furnished with a well-developed rim extending inward and 



