CRINOIDS. 149 



dotted lines indicate the relalioii of the different types accordinf? 

 to the evidence at present known, and probably coincide very 

 closely with the real courses of divergence. The scheme is, 

 then, to represent in a graphic way the relationship of the 

 genera as now understood, rather than to construct a genealo- 

 gical tree, with which attempts of this kind are often con- 

 founded. In the present instance, some of the earlier, more 

 generalized forms have not been made known as yet. There 

 are also good grounds for believing that some of the generic 

 types are considerably older than actual observation shows. 

 In other groups, more particularly, there is abundant evidence 

 pointing to a much higher antiquity of the leading generic 

 types than is generally supposed. This is especially true of 

 many widely distributed living organisms whose ancestry has 

 lately proved to be very ancient. 



The most generalized type of the family Actinocrinidsehas 

 dorsally a single ring of basal plates, three in number, and of 

 equal size, succeeded by a second circle of subequal pieces, 

 six in number — the five radials and the primary anal plate. As 

 in all the camerate crinoids, the brachials for a considerable 

 distance are incorporated into the calyx by means of interra- 

 dial ossicles, and in the free portion of the rays they are bise- 

 rial and closely interlocking. Yentrally five orals may, with a 

 few exceptions, be made out ; they are usually surrounded by 

 a greater or less number of smaller pieces. The anal aperture 

 may be a simple opening immediately back of the orals, or at 

 the end of a long ventral tube. The fundamental modifications 

 in the arrangement of the various plates give trustworthy cri- 

 teria for the basis of genera ; while the ornamentation, relative 

 size and shape of the calyx ossicles form very satisfactory 

 features for the distinction of species. The taxonomic values 

 attached by different paleontologists to the various characters 

 are not the same. This difference in interpretation, however, 

 appears to arise largely from the ontogenetic history of the 

 living forms of the class. But this diversity of opinion, hap- 

 pily, is rapidly lessening, with the prospect of a speedy agree- 

 ment, at least in the main features, as to the relative worth of 

 the separate structures in classification. 



