MONOGRAPH OF THE EXISTING CRINOIDS. 



109 



border of which is attached to the distal edge of the third brachial instead of to 

 the distal edge of the first, so that the larval arms are discarded at the first syzygy 

 instead of at the preceding synarthry, as in the case of multibrachiate species in 

 which all the division series are 2. 



Correlated with a high state of specialization in other structures, they appear 

 themselves to represent a high state of brachial specialization, and to be a consider- 

 able phylogenetic advance over the division series composed of two ossicles only. 



In all the species in which they occur they are relatively unstable, and are 

 subject to more or less extensive replacement by division series of two ossicles; in 

 fact, in most forms in which the IIBr series are normally 4(3+4) it is a relatively 

 rare circumstance to find a specimen in which at least one of the IIBr series is not 2. 



Division series of 4(3+4) are 

 relatively rare beyond the IIBr 

 series, for in a majority of the 

 species in which IIBr series of 

 4(3+4) occur the IIIBr series are 

 mostly or entirely 2. This is well 

 illustrated in the genera Crino- 

 metra (figs. 679-681, p. 338), Cros- 

 sometra (figs. 673, 675, p. 335), 

 Perissometra, Pachylometra (fig. 

 672, p. 335), Glyptometra, TTialas- 

 sometra, Zygometra (fig. 143, p. 

 83), Craspedometra (fig. 141, p. 

 83, and part 1, fig. 85, p. 139), 

 Heterometra, and Catoptometra. 



Sometimes beyond the IIIBr 

 series of 2 there are IVBr series 

 of 4(3+4) resembling the IIBr 

 series, and there may be a con- 

 tinuation of this alternation. This 

 is illustrated by species of Himero- 

 metra and Zygometra, but more 

 especially by the species of Comantheria where it reaches its greatest perfection in 

 the form known as Comantheria aUernans (fig. 169, p. 86). 



In Himerometra (fig. 123, p. 79), Craspedometra, and in the large species of 

 Zygometra (fig. 136, p. 79) the IIIBr series are usually 2 on the inner branch from 

 each IIBr series, and 4(3+4) on the outer. The IVBr and following series, if 

 present, are indiscriminately 4(3+4) or 2, usually with a majority of the former. 



In Comanthina schlegelii (fig. 138, p. 79) the conditions in the IIIBr series are 

 reversed, the inner being 4(3+4) and the outer 2; but on the posterior arm the 

 inner are commonly 2 and the outer 4(3+4) as in Himerometra. 



In genera and species characterized by the presence of IIBr series of 4(3+4) 

 in which IIIBr series occur these latter are often much more stable and more re- 



Fio. 189. LATERAL VIBW OF TYPE SPECIMEN OF EUDIO- 



CEINUS GEACILIS. 



