212 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



220-222, pp. 168-173) and T Jialassometra, the dorsal spinelets are arranged in 

 regular longitudinal lines, but usually, as in the Comasteridse, they are studded 

 thickly over the dorsal surface, at first small and fine, but becoming larger and 

 coarser distally. In certain of the Thalassometridse, as in the genus Asterometra, 

 there is a tendency for these spines, when developed on the pinnulars, to become 

 segregated along the crest of the dorsal ridge. 



In the Comasteridse all the postradial elements beyond the first brachial pair 

 are thickly covered with small spines, and this character is of such constancy as to 

 be of considerable diagnostic value, so that single brachials or pinnulars may 

 usually be determined as of species of that family by it alone. In the other groups 

 it is rarely well developed, and when present it is commonly confined to the 

 brachials, and usually to the outer half of their dorsal side, not occurring, or 

 being but imperfectly developed upon, the pinnulars and the proximal half of the 

 brachials. It is most prominent in species with prismatic pinnules. 



The spinosity of the elements of the first brachial pairs, of the elements of the 

 division series, and of the radials, is apparently of an entirely different type and is 

 of very great systematic significance. Whereas the spinelets on the surface of the 

 outer brachials and of the pinnulars are directed anteriorly and are much better 

 developed on the outer part of the segments than on the inner, those on the proximal 

 ossicles and on the radials are erect, distributed uniformly over the plates or form- 

 ing borders around the edges of the dorsal surface which are of the same width 

 proximally as distally and are composed of spinelets of equal size. 



In the genera Oceanometra (figs. 220-222, pp. 168-173), Thalassometra (figs. 

 223, 224, pp. 175, 177), Stylometra (fig. 219, p. 166), Sarametra (part 1, fig. 109, p. 

 175), and Zenometra the radials, division series, and lower brachials are always 

 covered with fine spines, these furnishing valid generic and specific characters of 

 the highest importance. Typically these spines are uniformly developed all over 

 the dorsal surface of the ossicles, but in certain species they are much larger along 

 the borders. In the genus Crinometra (figs. 678-683, p. 338) the radials, division 

 series, and first brachial pairs are usually thickly covered with small spines or 

 tubercles of various sizes. These may be arranged in horizontal rows, as in 

 C. gemmata (fig. 679, p. 338), or may be quite without arrangement; they may be 

 entirely and well separated, or confluent. 



The species of Mariametra (figs. 196, p. 124, 197, p. 125, and 684, p. 338) also 

 have spinous radials, division series, and first brachial pairs, but the center of the 

 ossicles is always smooth and obscurely carinate, the keel being sometimes more or 

 less tubercular. A similar condition also occurs in certain species of Cosmiometra 

 and of Crinometra. 



Like the development of spinosity on the dorsal surface of the postradial 

 ossicles, the development of a median dorsal carination appears to be divisible into 

 two types, each quite distinct from the other, though both often occurring together, 

 and both reaching their maximum in the same groups. In one type only the outer 

 brachials are affected, from the first or second syzygial pair onward (more rarely 

 only the distal brachials), and in the second only the division series and first 



