. Bulletin - United States National Museum. Science. REPORT ON THE ECHINOIDEA MORTENSEN" 247 The primary spines at the ambitus are not very long, the longest preserved being only about 100 mm. in length by a thickness of mm., not tapering. Possibly there may have been a few slightly longer, but it may be said with certainty that the longest can only very slightly have exceeded 1 Yi h. d. In the present specimen the upper spines taper to a point and are much shorter than those at the ambitus, though fully developed, as is evident from the character of their surface, which means that th
. Bulletin - United States National Museum. Science. REPORT ON THE ECHINOIDEA MORTENSEN" 247 The primary spines at the ambitus are not very long, the longest preserved being only about 100 mm. in length by a thickness of mm., not tapering. Possibly there may have been a few slightly longer, but it may be said with certainty that the longest can only very slightly have exceeded 1 Yi h. d. In the present specimen the upper spines taper to a point and are much shorter than those at the ambitus, though fully developed, as is evident from the character of their surface, which means that the specimen has reached its full size. The spines are perfectly smooth, shining, at most with a pair of in- distinct longitudinal ridges, without any serrations. Even under the microscope no longitudinal striation is to be observed. The collar is short, 3-4 mm. in length, widening toward the milled ring. The oral primaries are of the typical form, curved, strongly serrate, with a smooth point, ending abruptly, not bifid as may be the case in some species (elegans, acutispina); but this may be due to the point being worn. The two proximal ones are fairly broad, the third is more slen- der, and the fourth is only recognizable as an oral pri- mary through its strong lateral serration; the fifth and sixth still carry some small lateral serrations, being transitional forms. From the third onward the spines have the point slightly widened, longitud- inally serrate, ending ab- ruptly as if worn off. This also holds good for the longest spines at the ambitus, which are directed downward, partly even slightly curved. Evidently the animal has been walking on the points of all these spines, only the shorter spines above the ambitus being directed upwards. * The secondary spines surrounding the primaries are fairly robust, about 12 mm. long, broad, distinctly excavate on the outside, espe- cially toward the end, which is not at all or only slightly narrowed. (PI. 76, figs. 1-2.) This excava
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