MONOGRAPH OF THE EXISTING CRINOIDS. 



157 



smooth the distal pinnules will be smooth, but if the brachials possess overlapping 

 and spinous distal ends, then the distal ends of the third and following pinnulars 

 will also be armed with spines. If there are 10 arms the pinnules will be relatively 

 long, with proportionately long component segments; but if the number of the 

 arms be more than 10 the pinnules become proportionately shorter and the length 

 of the component segments becomes propor- 

 tionately less and less. 



As a general rule the species with 10 

 arms only have longer distal pinnules than 

 multibrachiate species, and the relative length 

 of the pinnules decreases in direct correla- 

 tion with the increase in the number of arms. 



It may be said also that the species of 

 the Macrophreata possess longer distal pin- 

 nules, composed of longer segments, than 

 those of the Oligophreata, though, taking 

 the 10-armed species of these two groups as 

 a whole, the difference is not great. The as- 

 sumption of a prismatic form as seen in the 

 Calometridae, Thalossometridae, and Charito- 

 metridse is accompanied by a more or less 

 marked shortening of the pinnule, which 

 again becomes elongated whenever, as in cer- 

 tain species of the Charitometridse, the pris- 

 matic character becomes more or less obso- 

 lescent. 



Among the species of the Macrophreata 

 there is little deviation in the character of 

 the distal pinnules from a common type; they 

 are always greatly elongated, cylindrical, ex- 

 ceedingly slender, with expanded articula- 

 tions. As the two basal segments are not 

 affected by any change in the outer segments, 

 they appear unusually large or laterally 

 swollen and contrast strongly with the slen- 

 der segments succeeding. 



The systematic importance of the dis- 

 tal pinnules is very slight. In general they 

 are not consulted except to ascertain the presence or absence of side and covering 

 plates, which is rather more readily determined upon them than upon the other 

 pinnules ; but the characters in the earlier pinnules, brachials, arm bases, and cirri 

 always correlated with the presence of an ambulacra! plating, are so marked that 

 after a little experience one never refers to the distal pinnules to determine this 

 point. In the Comasteridse they are very spinous dorsally and therefore diag- 

 nostic, but here, because of the prominence of the terminal combs present on the 



FIG. 214.- 



LATEHAL VIEW OF TYPE SPECIMEN OP 

 COSMIOMETRA DELICATA. 



