A. H. ClarJc — Recent Crinoid Faunas. 131 



Occasionally faunas are found which combine the character- 

 istics of two or more of the stages described above ; these are 

 rejuvenated faunas, faunas which have progressed to the 

 extreme point indicated (or perhaps slightly further) and then 

 have been subjected to some change in environmental condi- 

 tions which has served as a stimulus, and set a greater or 

 lesser part of the fauna some distance back along the phylo- 

 genetic fannal path. 



Such a fauna we find indicated by the crinoids about the 

 shores of the large East Indian Islands ; some of these approach, 

 in the exaggeration of their specific characters, the Australian 

 species, while others are very generalized, with several closely 

 related species. The crinoid fauna of western Europe is also 

 a rejuvenated fauna; it is composed of two genera, Antedon 

 and Leptometra, both very close to East Indian genera, the 

 former with five species, three highly specialized and two 

 primitive, the latter including two species, one specialized and 

 one more primitive ; in each case the more primitive species 

 occur in the Mediterranean. 



It is possible to analyze a fauna on the basis of a single char- 

 acter in a group. Let us take, for instance, the character of 

 the centrodorsal in the Comasteridae. This organ differs in the 

 several genera and species composing the family only in the 

 degree of specialization, the developmental lines being every- 

 where the same. In some species, as in Comanthus bennetii, 

 the centrodorsal always remains essentially as in the younsr, 

 but increases in size throughout the life of the individual. 

 Usually, however, resorption takes place at the dorsal pole 

 which is gradually planed off, as it were, so that the centrodor- 

 sal changes from the primitive hemispherical form and becomes 

 discoidal, the rows of cirri dropping off as the sockets are 

 resorbed. In extreme cases the resorption results in reducing 



I the centrodorsal to a thin stellate plate, without any traces of 

 cirrus sockets, countersunk within the center of the dorsal sur- 

 face of the radial pentagon. 

 We may arrange all comasterid centrodorsals in a linear 

 series, calling the least developed {Comanthus bennetti) type 

 A, and the atrophied stellate disk D, B, and C denoting inter- 

 : 



mediate stages. 



Now the species of the Australian fauna have centrodorsals 

 which run from A to D, but with especial emphasis on the D ; 

 the species of the East Indian fauna also run from A to D, but 

 the emphasis is between B and C ; the Japanese species run 

 from A to C, with especial emphasis at B ; the West Indian 

 and east African species are confined between B and C. This 

 holds good regardless of the subfamily or genus to which the 

 species may belong, and exactly the same thing may be worked 

 out in regard to other characters in this family, and with other 

 characters in other families. 



