304 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



overlapping edges of the segments may be pointed dorsally; the penultimate seg- 

 ment differs but little from the preceding, and almost always bears a terminal or 

 subterminal opposing spine, which, however, is never strongly developed; the 

 terminal claw is slender, never especially long, and always tapering evenly, and 

 evenly curved. 



There are two lines of departure from this general type. One (which finds a 

 parallel in the Atelecrinida3 and Pentametrocrinidse) is in the direction of an elonga- 

 tion of the segments, especially distally, coupled with an increase in their number 

 and an excessive lateral flattening which extends far inward toward the base of 

 the cirri, and with the suppression of the opposing spine and great reduction and 

 straightening of the terminal claw; this reaches the maximum in TTiysanometra 

 (fig. 372, p. 299), and is to be noticed in various degrees of perfection in the species 

 of Psathyrometra (fig. 379, p. 301), Thaumatometra, Iridometra, Compsometra, and 

 Coccometra (figs. 374-376, p. 299). In Leptometra (figs. 381, 382, p. 301, 383, p. 303, 

 and 384-386, p. 305), which is an offshoot from the Psathyrometra stock, this con- 

 dition has been carried to an extreme; but it has here been masked by an absence 

 of the reduction in the size of the cirri, whereby the expansion of the ends of the 

 segments and the characteristic lateral flattening have become more or less obso- 

 lete, the cirri as a whole tending toward the condition seen hi Craspedometra (fig. 

 85, p. 139). 



The elongation of the cirri may, however, be brought about in an entirely 

 different manner; the cirri at first may consist of some half dozen elongated 

 segments, the number gradually increasing in the subsequent cirri until sometimes 

 as many as 80, or even more, may be found in the longest. But the added seg- 

 ments do not resemble the earlier ones. The six segments of the cirri of the young 

 animal are repeated in all the subsequent cirri without change; the additional seg- 

 ments are added progressively at the distal end of the later cirri, and they are 

 progressively shorter and shorter until a minimum length is reached, which is 

 usually about equal to the transverse diameter, after which all the added segments 

 are the same. Cirri of this type (which merely differs from the type characteristic 

 of the Thalassometridas in that the short segments are added gradually instead of 

 with phylogenetical suddenness) may be at once recognized by having the proximal 

 portion made up of elongated segments and the distal of a greater or lesser series 

 of short segments of equal size. Such cirri are found in Perometra (fig. 387, p. 307), 

 Eryfhrometra, Balanometra, Zenometra (more like those of the Thalassometridse here) 

 (figs. 109, p. 175, and 377, 378, p. 301), Adelometra (fig. 380, p. 301), Heliometra (fig. 

 392, p. 307), Solanometra, Anthrometra, and Florometra (fig. 391, p. 307), Promacho- 

 crinus, certain species of Coccometra and of Iridometra, Hafhrometra, Trichometra, 

 certain species of Bathymetra (fig. 402, p. 311), Hypalometra (fig. 388, p. 307), and 

 Nanometra (fig. 390, p. 307). In Perometra and in Zenometra we find the same factor 

 obscuring the general plan that was noticed in Leptometra; for the cirri have 

 become stout, so that in some cases the normal central constriction of the long 

 earlier segments has disappeared, the cirri are less compressed distally, and the 

 outer segments are much shorter than usual and are produced and strongly cari- 

 nate dorsally, just as in such genera as Asterometra (figs. 94, p. 155, and 362, p. 295), 



