l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



great depths, as in the Arctic, the Antarctic, the Mediterranean, etc., 

 the httoral crinoids descend with it and there is then a uniformity 

 in the crinoid fauna from the surface almost or quite to the abysses ; 

 but where the temperature decreases rapidly with depth, as along the 

 tropical shores, the bathymetric extension of the littoral types is very 

 limited and the littoral fauna is underlain by an intermediate fauna 

 consisting for the most part of more ancient and more conservative 

 types which is fairly uniform the world over, though it reflects the 

 broader divisions of the local overlying faunas. It is from this inter- 

 mediate fauna that the littoral faunas on the one hand and the abyssal 

 faunas on the other appear to have been derived. 



The distribution of the crinoids appears to be governed entirely by 

 temperature, pressure playing no part whatever. 



There seems to be a close correlation between the bathymetric (or 

 thermal) range and the geographical range of the same type, a form 

 with a, restricted bathymetrical range having a similarly restricted 

 geographical range and the reverse. 



THE PALEONTOLOGICAL HISTORY OF THE LIVING CRINOIDS 



Of the four great divisions of the crinoids, the Camerata, the 

 Flexibilia, the Inadunata and the Articulata, two, the Inadunata and 

 the Articulata, are represented in the present seas. 



The Inadunata are almost entirely confitied to the Paleozoic, rang- 

 ing from the Ordovician to the Carboniferous, with one of the i8 

 iiamilies (Poteriocrinidse) represented in the Trias and another 

 (Plicatocrinidse) known only from the Upper Jurassic and from the 

 recent seas. 



The Plicatocrinidse (including the living genera C alamo crinus, 

 Ptilocrinus, Thalassocrinus, Gephyrocriniis and Hyocrinus and the 

 fossil genus Plicatocrinus) are at present entirely confined to the deep 

 and cold abysses ; all of the species are very rare, most of them having 

 only been dredged once and none of them more than twice. The 

 Jurassic representative of this family is also very rare, and its remains 

 are commonly associated with those of hexactinellid and lithistid 

 sponges so that it also probably lived at considerable depths. No 

 representatives of the Plicatocrinidse are known from the Cretaceous 

 or from the Tertiary. 



The Articulata are entirely post-paleozoic, all from the Jurassic or 

 later except the pentacrinites, which are also represented in the Trias. 



The Phrynocrinidje are not represented as fossils ; the ranges of 

 the other families and of the genera including both recent and fossil 

 species are as follows : 



