MONOGRAPH OF THE EXISTING CRINOIDS. 17 



they exhibit any strong tendency toward dissociation of ordinarily correlated char- 

 acters; but the sudden and much more abrupt departure from the normal cruioid 

 habit seen ui the comatulids has been accompanied by, or the entirely new conditions 

 under which they live and the consequent extraordinary atrophy of their calyx have 

 induced, the development of all sorts of structural variants and excesses which have 

 not yet had time or, because of the passive part the animals play ui their relations 

 to other animals, have not yet been forced, to crystallize into definite types with a 

 definite scheme of correlation. 



The morphological diiTcrence between the pentacrinites and the comatuUds is 

 merely that the weakerung of the syzygial union between the first nodal formed 

 and the infranodal just below it in the comatulids leads to its rupture before any 

 additional segments are formed, while in the pentacrinites rupture does not occur 

 until many other columnars have been intercalated between this nodal and the 

 calyx. The pentacrinites thus continue to build a long, many-jointed stem, while 

 the comatulids condense the entire stem withm the compass of the fu-st-formed 

 nodal. The morphological difference between the comatulids and the pentaciinites 

 reduced to its lowest terms therefore is merely a slight difference in the develop- 

 ment of the tendency to rupture at the syzygy between the first-formed nodal and 

 the columnar just beneath it. 



The comatulids and the pentacrinites occupy a curiously anomalous system- 

 atic position, for both groups are far removed from the direct line representing the 

 progressive phylogenetical development of the class. But both, though widely 

 divergent, agree in differing from all other related types through discarding the proxi- 

 mal portion of the column and in the development of a highly cirriferous proximale, 

 which in the pentacrmites is indefinitely reduplicated. 



The genus ThioUiericrlmts occupies a position midway between them; species 

 of this genus develop a cirriferous proxunale, but retain the larval column; the 

 relation of ThioUiericrinus to the pentacrinites and to the comatulids may roughly 

 be graphically expressed by the foUowmg formula: 



pentacrinites + comatuhds mr-n- • ■ 

 c = ThioUiencnnus. 



ThioUiericrinus, however, is in the direct Ime rcpresentmg the progressive 

 phylogenetical development of the class, and approximates very closely, if it does 

 not actually represent, the type from which, by sudden diametrically opposite 

 deviation, both the pentacrinites and the comatulids have been derived. 



Systematically the pentacrinites, TTiioUiencrinus and the comatulids repre- 

 sent a small group of which ThioUiericrinus is the true phylogenetical exponent, the 

 other two types being aberrant departures from this stock. 



TJiioUericrinus is fossU only. In the recent seas the comatuhds far outmmiber 

 all of the other crinoids taken together, at the same time extendmg through a much 

 wider geographical, bathymetrical and thermal range, while by far the largest of 

 the remaining groups is that of the pentacrinites. 



These two highly aberrant types therefore dominate the recent seas, and so 

 pronounced is their dominance that when compared with them all the other types 

 become relativelv insignificant. 



