14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J2 



Asymmetry is equally characteristic of the genus Thaumatocrinus, 

 the most specialized genus of the family Pentametrocrinidse ; in this 

 genus the third type is found. 



Asymmetry exists in all of the recent genera of the Plicatocrinidse, 

 the first, second and fourth types being represented ; the third also 

 occurs in the fossil representatives of this family. 



Asymmetry is characteristic of both of the recent genera of 

 Apiocrinidse, which are the most highly specialized genera in the 

 family ; in these the second type occurs. 



Asymmetry of the second type is characteristic of Holopus. 



Asymmetry characterizes both of the species of Rhisocrinus exist- 

 ing in the present seas, and one of the species of Monachocrinus; in 

 these the third type is found. 



It appears that, no matter in what form it may manifest itself, in 

 the recent crinoids asymmetry is an attribute of the most special- 

 ized types in the groups in which it occurs. From the conditions in 

 the Plicatocrinidse, the last remnants of the once abundant Inadunata, 

 it would appear that asymmetry is an attribute of phylogenetically 

 decadent types — types in which type senescence has so far advanced 

 as to inhibit the normal course of development. 



Although occurring everywhere except in the Arctic Ocean and in 

 the Mediterranean, Bering, Okhotsk and Japanese seas, asymmetrical 

 types are most frequent and most highly developed ( i ) in warm and 

 shallow water from southern Japan southward throughout the 

 Malayan archipelago to northern Australia and thence westward to 

 Ceylon, and (2) in the Antarctic and in the cold abysses. 



Though present among species inhabiting the western Atlantic 

 from North Carolina to Brazil, and characteristic of many forms 

 living at intermediate depths in the western Pacific and in the Indian 

 Ocean, in these it is never more than slightly developed, even though 

 they may be very closely related to types in which it is, in other situa- 

 tions, carried to an extreme. 



Briefly stated among the recent crinoids any wide departure from 

 the normal close approximation to true pentamerous symmetry indi- 

 cates unfavorable conditions of one or other of two main types which 

 are not mutually exclusive ; these two types are : 



1. Internal unfavorable conditions, induced by incipient phylo- 

 genetical degeneration through type senescence, as in the Plicato- 

 crinidse which in the recent seas represent the otherwise almost ex- 

 clusively palaeozoic Inadunata ; and 



2. External unfavorable conditions, taking the form of 



a. Phylogenetically excessive cold, and 



b. Phylogenetically excessive warmth. 



