80 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



It is in the families Thalassometridse and Charitometridse that we find the 

 greatest variability in the number of arms between the component genera, and 

 apparently also in the different species. 



In the family Thalassometridae (figs. 124, 145, 165, 203-225, pp. 79, 83, 86, 139- 

 179) the species of the genera Aglaomttra (fig. 225, p. 179) and Stiremetra have 10 

 arms only, those of the genera Asterometra (fig. 209, p. 149), T fiala.ssometra (figs. 

 223, 224, pp. 175-177), Daidalometra (fig. 212, p. 153), and Pammetra (figs. 217, 

 218, pp. 163-164), 10 or more, and those of the genera Ptilometra (fig. 204, p. 139, 

 and pi. 53, fig. 1346), Pterometra (fig. 203, p. 137), Horceometra, Oceanometra (figs. 

 145, 220-222, pp. 83, 168-173), Stylometra (fig. 219, p. 166, and pi. 53, fig. 1345), 

 Lissometra (fig. 216, p. 161), Crotalometra (fig. 210, p. 150), and Cosmiometra (figs. 

 213-215, pp. 155-159), always more than 10, ranging from fewer than 15 in Lisso- 

 metra to between 20 and 30 in Oceanometra and Stylometra. The division series, 

 however, are always all of two ossicles except in the species of Thalassometra and of 

 the closely related Oceanometra, where the IIBr series are usually, though not 

 always, of four. As the species of T ' halassometra may have 10 or more arms it 

 happens that single species are found to be represented by individuals which fall 

 into three of Carpenter's groups, the " Basicurva group," the " Spinifera group," 

 and the " Granulifera group." 



In the Charitometridse (figs. 122, 198, 199, pp. 79, 127, 129) the species of the 

 genera Strotometra (figs. 230-232, pp. 188-192), Pwcttometra (fig. 229, p. 186), 

 Charitometra, Chondrometra (part 1, fig. 99, p. 160), and Calyptometra (fig. 676, 

 p. 338, and pi. 54. fig. 1347) have 10 arms only, those of the genera Perissometra (fig. 

 227, p. 182, fig. 658, p. 329, and fig. 670, p. 335) and Chlorometra (fig. 228, p. 184) 10 

 or more, and those of the genera Crinometra (figs. 678-683, p. 338), Crossometra 

 (figs. 226, p. 180, and 673, 675, p. 335), Monachometra, and Pacfiylometra (figs. 671, 

 672, p. 335) always more than 10, in the case of Crossometra from 26 to 33; in this 

 family the IIBr series, when present, are rarely 2, being almost invariably 4(3+4), 

 at least in the majority of cases. 



With only two exceptions, outside of occasional sporadic instances, the post- 

 radial series of the species belonging to the Macrophreata divide once or, more 

 rarely, not at all, so that the great majority of the forms in this group have 10 

 arms, two arising from each radial; a few 5 arms, one being borne by each 

 radial ; a few 10 arms, one on each of 10 radials ; and one 20 arms, two arising 

 from each of 10 radials. 



Perometra afra, of the subfamily Perometrinse, has from 11 to 14 arms with 

 all the division series 2, and the only known specimen of Adelometra angustiradia, 

 belonging in the subfamily Zenometrinse, has 13 arms with the three IIBr series 

 present 4, 4(3+4) and 2. 



In Antedon bifida several specimens with more than 10 arms have been re- 

 ported, most of these having 11, and one of them 12. The IIBr series are usually 

 2, but occasionally 4(3+4). 



In one macrophreate family, the Pentametrocrinidse (figs. 126, p. 79, and 1016. 

 pi. 6), division series are not developed at all, so that the arms are undivided, 

 5 or 10 in number according to the number of the radials. These undivided arms 



