A. II. Clark — Recent Crinoid Faunas. 



129 



Although the Bering Sea fauna is the nearest approach to a 

 young fauna which can be found, yet there are a few distinct- 

 ive genera which show an approach to the next epoch of faunal 

 existence. 



So far as the crinoids are concerned, the antarctic region is 

 very young ; here we have Solanometra and Promaehocrinus 

 (the latter merely differing from it in the doubling of all the 

 radials), each with several very variable species, though none 

 so variable as the Bering Sea representatives of the same group. 



Adolescent faunas exhibit a comparative stability of specific 

 types, coupled with the incipient formation of new genera as a 

 result of a growing tendency of the species to depart widely 

 from the generic mean. 



The crinoid fauna of southern Japan might be considered as 

 an adolescent fauna; here we find many genera including 

 several species, each very stable and showing comparatively 

 little variation, such as Catoptometra, Comanthus, Pichrome- 

 tra, Parametra, Pectinometra, Thaumatometra, and Penta- 

 metrocrinus, while Erythrometra, Nanometra, Calometra, 

 Carpenteroerinus and Phrynocrinus are not known elsewhere, 

 though the two last, being from deep water, probably occur to 

 the southward. 



In mature faunas the species are fixed, save only for the 

 species at the mean of each genus, which always remains vari- 

 able, and new generic types are found which have become 

 separated off from the parent genera through the suppression 

 of intermediates, or have arisen by discontinuous variation. As 

 a result of the formation of these new generic types, the num- 

 ber of species in each genus is diminished, and the species are 

 found to approach more or less closely the means of the origi- 

 nal genera, or the means of the genera newly formed. 



The West Indian crinoid fauna appears to be approximately 

 a mature fauna. It contains a number of peculiar genera, 

 such as Hypalometra, Coccometra, Zeptonemaster, Comatilia, 

 Microcomatula, and Analcidometra, while almost all of the 

 East Indian genera which occur here have become more or less 

 differentiated from the original stock, forming new genera 

 parallel to the original East Indian types. Tn several cases 

 single East Indian genera have given rise to two or more 

 West Indian genera, as Comissia, from which Comatilia, 

 Microcomatula, and Leyytonemaster appear to have been 

 derived, as well as two other genera which up to now have 

 remained undescribed. 



Senescent faunas have lost a considerable proportion of the 

 genera which they possessed at maturity ; the genera which 

 remain include aberrant species in which certain characters 

 have become greatly exaggerated, giving those species a curi- 

 ously unbalanced appearance. There is typically but a single 



