214 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



brachial pairs, together with the radials. The first type is always accompanied by 

 a strong lateral compression of the brachials, and the caruiation has a tendency to 

 lean anteriorly, taking the form of a more or less broad overlapping spine. The 

 second type is, on the other hand, accompanied by a considerable broadening of the 

 plates affected, and the carination is perfectly erect. 



Carination of the distal brachials does not occur unless those segments are 

 smooth dorsally; but carination of the division series and first two brachials is 

 almost invariably accompanied, as in Crinometra (figs. 678, 680, 683, p. 338), 3/a/v'a- 

 Tnetra (fig. 684, p. 338), Glyptometra, P€rist<ovictra (fig. 658, p. 329), Cotylometra 

 (fig. 199, p. 129), Oceanometra (figs. 220-222, pp. 168-173), and Thalassomstm 

 (fig. 224, p. 177), by a spinous or tubercular modification of the dorsal surface. 



In the Comasteridse a very thin, narrow, low, and inconspicuous median eleva- 

 tion which can be detected only with difficulty occurs more or less constantly on 

 the brachials of certain species of Comatida, and in Comatonia a few of the lower 

 brachials have the central portion elevated into a blunt keel with a spinous crest. 

 Except for these cases neither type of carination is ever found in the Comasteridse 

 other than as a rare variant, nor in the Zygometridse, Himerometridae, or Stephano- 

 metridse. Proximal carination occurs in all the species of Mariametra (figs. 196, 

 p. 124, 197, p. 125, and 684, p. 338), though never found in those of Dichrometra, 

 Oxymetra (fig. 195, p. 122), or Pontwnietra (fig. 194, p. 120), and in Cotylovi-etra 

 omata (fig. 199, p. 129), though not in C. gracilicirra nor elsewhere in the Colo- 

 bometridse. In the Tropiometridae distal carination is characteristic of all the 

 smaller species of Tropiometra (fig. 173, p. 89). but is not found in the two largest. 

 It occurs in all of the species of Calometridse, in one genus of which there is also a 

 very slight proximal carination in a few of the species. 



It is in the Thalassometridse that both types attain their maximum develop- 

 ment. In the Ptilometrinse the distal carination is excessive, though usually it does 

 not occur on the lower part of the arm ; but certain species of Asterometra (those 

 allied to A. longidrra [figs. 206, 207, pp. 143, 145]) and of Pterometra (those allied 

 to Pt. trichopoda [fig. 208, p. 146]) may possess very strongly marked proximal 

 carination, this reaching its maximum in some of the species of the former (figs. 

 206, 207, pp. 143, 145), though entirely absent from others (fig. 209, p. 149, and part 

 1, fig. 94, p. 155). In Stiremttra and in C'osmiomefra (figs. 213, 214, pp. 155, 157) 

 only proximal carination occurs, in the latter being only faintly evident. In 

 Stylovietra (fig. 219, p. 166, and part 1, fig. 35, p. 73) only distal carination is found, 

 here, however, reaching its greatest development. In Parametra both types are 

 found, the distal, except in one species (fig. 218, p. 164), strongly marked, the 

 proximal scarcely noticeable. In TKalassometra both types are found, though 

 never in the same species. In Oceanometra gigantea (fig. 220, p. 168) the proximal 

 carination is curiously restricted to the second element of the first brachial pairs 

 and to their morphological reduplications, never occurring on any of the other seg- 

 ments. The species of Stenometra (fig. 211, p. 152) and Daidalometra (fig. 212, p. 

 153) have all the postradial ossicles very strongly carinate. 



