78 BUIXETIN 82, UNITED STATES NATIONAL, MUSEUM. 



POST-BADIAL ELEMEKTS. 



General features. 



Following the radials there is in all recent crinoids a regularly linear series of 

 ossicles which usually forks, giving rise to two similar series, on the second (figs. 

 183-185, pp. 98-102) ; each of these secondary series may again fork, giving rise to 

 four similar series (figs. 181, 186, pp. 94, 104) , and the process may be repeated, in 

 extreme cases, as many as eight or nine times (figs. 180, 188, 194, pp. 92, 108, 120). 



The number of the arms has always been used as a basic character for the 

 differentiation of the comatulid species since the days of Linck and Seba. P. H. 

 Carpenter in his monograph divided his " series " of '■'■Antedon " and "Actinometra " 

 into various groups according to the number of arms and the number of ossicles 

 found in the division series. He believed that there was a definite line of demarka- 

 tion between species with 10 arms and those with more than 10, and between species 

 with the IIBr series of two elements and those with the IIBr series of four. He 

 was forced to admit, however, the existence of several species which fell into two, 

 or even three, of his groups through being both 10-armed and multibrachiate, and 

 through having the IIBr series sometimes of two and sometimes of four ossicles. 

 In regard to the number of the component ossicles of the IIBr series he admitted 

 a certain amount of variability, but placed the species in one group or in another 

 on the basis of a majority of their series. This procedure resulted in the inclusion 

 of Comanthus parvicirra (under dift'erent names) both in the " Valida group " and 

 in the ^^ Parvich'ra group" of '■'■Actinometra.''^ 



The availability of the number of arms and of the number of the ossicles in the 

 division series for systematic purposes varies very greatly in the different groups. 

 Speaking broadly, the number of arms may be said to be a systematic index of 

 prime importance, but at the same time one which must be used with considerable 

 caution. 



In the Comasterida; (figs. 180-188, pp. 92-108) the number of arms is, as a 

 rule, perfectly true to species within relatively narrow limits and, taken in connec- 

 tion in multibrachiate genera with the second, or third, division series, furnishes the 

 most obvious differential criterion, and one that may be used with perfect confi- 

 dence; for no comasterid is known which is both 10-armed and multibrachiate, 

 and no comasterid has ever been found which is variable in both the second and 

 third division series, though many are more or less variable in one or the other. 



Even more fixed are the number of arms and the types of division series in 

 the species of Himerometridas (figs. 123, 141, 192, 193. pp. 79, 83, 115, 117), Ste- 

 phanometridae (figs. 125, 130, p. 79), Mariametridse (figs. 191-198, pp. 120-127), and 

 Colobometridte, and in most of those of the Zygometrida^ (figs. 127, 136, 143, 189- 

 191, pp. 79, 82, 109-112), furnishing, so far as they go, the best systematic characters 

 to be found ; but their availability here is restricted by the relatively slight vari- 

 ability within each family. 



Practically the same is true in the Calometridaj (figs. 200-202, pp. 130-134) , but 

 in one of the genera of this family {C alometra) species occur which are both 10- 

 armed and multibrachiate. 



