80 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



It is in the families Thalassometridse and Charitometridae 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 ThalassometridiB (figs. 124, 145, 165, 203-225, pp. 79, 83, 86, 139- 

 179) the species of the genera Aglaometra (fig. 225, p. 179) and Stlremetra have 10 

 ai-ms only, those of the genera Asteromet.ra (fig. 209, p. 149), T halassometra (figs. 

 223, 224, pp. 175-177), Daidal&metra (fig. 212, p. 153), and Paranxetra (figs. 217, 

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

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

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

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

 213-215, 11]). 155-159), always more than 10, ranging from fewer than 15 in Lisso- 

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

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

 the closely related Oceanojnetra., 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), Paicilometra (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), M onachometra, and PachyJometra (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 majoi'ity 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. 



Perom£tra afra., of the subfamily Perometrinse, has from 11 to 14 arms with 

 all the division series 2, and the only known specimen of Adelom^tra angustlradia, 

 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 macro phreate family, the Pentametrocrinida; (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 



