L30 A. II. Clark — Recent Crinoid Faunas. 



species in each genus ; but there may be two or more, each 

 with a different set of characters exaggerated, in which case 

 they are usually treated as aberrant monotypic genera. 



The Australian crinoid fauna is a perfect example of a senes- 

 cent fauna. It includes about fifty species, nearly all of which 

 are remarkable for the grotesque exaggeration of their specific 

 characters. Even in certain wide-ranging forms, as Gomatula 

 Solaris, Australian specimens have their characters greatly 

 increased over those from other regions. 



Some examples of this exaggeration of the specific charac- 

 ters which make the Australian crinoid fauna unique are: the 

 secondary development of biserial arms, indicated in Comatu- 

 lella brachiolata, but not known outside of that genus, which 

 is confined to southern Australia; the development of exces- 

 sively short cirrus segments and pinnules, also peculiar to this 

 genus ; the development of very slender cirri in a comasterid, 

 only found in Gomanthus trichoptera from southern Australia ; 

 the development of twenty armed species of the normally ten 

 armed genus Gomatula ; the occurrence of heterotomous arm 

 division, unique among the comatnlids, in the same genus ; the 

 development of enormous processes on the pinnule segments, 

 seen in Comanthina belli and Oligometra carpenteri, represent- 

 ing two widely different families; the development of a very 

 large centrodorsal bearing cirri irregularly arranged in a genus 

 of Thalassometridse, Ptilometra ; excessive reduction of mus- 

 culature in the same family, illustrated by the same exclusively 

 Australian genus ; incipient suppression of side, covering, and 

 other perisomic plates, seen in the same genus ; extraordinary 

 development of interradial plates in adult comasterids, as seen 

 in Comanthina belli and Comaster multifida y complete plat- 

 ing of the disk, dissociated from the development of side and 

 covering of plates, as seen in Zygometra elegans, Z. microdis- 

 cus and Z. multiradiata ; greatly enlarged lower pinnules in 

 a zygometrid, as seen in Z. miciodiscus ; very small lower 

 pinnules in a zygometrid, as seen in Z. multiradiata / aberrant 

 zygometrid arm division, as seen in Z. elegans. 



A pathological fauna may resemble a senescent fauna in its 

 general facies ; but in a pathological fauna all the species, 

 besides being aberrant, are excessively variable, which is never 

 the case in a senescent fauna. Pathological faunas occur 

 usually on the limits of faunal areas, or on the boundary 

 between two very different faunal areas, and are composed in 

 the latter of intrusive species from both the adjacent areas. 

 Many of the animals in Massachusetts Bay, at least in the 

 southern part, indicate a pathological fauna. This is shown by 

 a study of the local species of almost any of the echinoderms, 

 which exhibit an unusually large proportion of abnormalities 

 and aberrant variants of all sorts. 



