MONOGKAPH OF THE EXISTING CRINOIDS. 



149 



these spines are longest at the ventral distal corners of the pinnulars, and where 

 the longitudinal ridge meets the distal border. 



In the Oligophreata there is sometimes to be observed a curious correlation 

 between the size of, and the number of segments in, the oral pinnules and the 

 cirri. Oomanthus pinguis and C. bennetti are both remarkable for the enormous 

 size of the oral pinnules as well as for the great size, stoutness, and number of 

 segments in the cirri, while in the allied C. solaster, C. trichopetra, C. samoana, etc., 

 both the oral pinnules and the cirri are small and slender with comparatively few 

 component ossicles. 



The systematic value of the oral pinnules varies very greatly in the different 

 groups. They present reliable diagnostic 

 characters for distinguishing the families 

 Thalassometridse, Charitometridse, Comas- 

 teridae, Calometridse, and Stephanometridse, 

 and the subfamilies Ptilometrinae, Thalasso- 

 metrinse, and Heliometrinse, and in combi- 

 nation with the middle pinnules the family 

 Tropiometridse. In these groups they are of 

 relatively slight value for the differentia- 

 tion of genera, and of still less for the dif- 

 ferentiation of species. In the remaining 

 families and subfamilies in which they oc- 

 cur in widely different forms their impor- 

 tance for the determination of genera and 

 often of species is very great. 



Middle or genital pinnules. 



Immediately following the oral pin- 

 nules there are usually from one to three 

 pairs of pinnules which are more or less 

 intermediate in character between the oral 209 



and the genital pinnules, lacking Well- FlG - 209 - LATERAL, VIEW OF TYPE SPECIMEN OB 



1 1 j 111- ASTEBOMETRA I.EPIDA. 



developed gonads and having a calcareous 



structure indicative of the former, though much more generalized, but possessing 



ambulacral grooves and nerves, lappets, and tentacles like the latter. 



As a rule there are two or three pairs (figs. 272, 273, p. 207) of these inter- 

 mediate pinnules in oligophreate species, and one (fig. 281, p. 215) or none (fig. 282, 

 p. 215) in the macrophreate forms. In the Macrophreata in most cases they re- 

 semble exactly the genital pinnules, but bear a smaller gonad or none at all ; usually 

 in the Oligophreata and in the very large macrophreate species, such as the species 

 of Heliometrinse (figs. 293. 294, p. 221), they are intermediate in length and struc- 

 ture between the two types. 



