MONOGRAPH OF THE EXISTING CRINOIDS. 17 



they exhibit any strong tendency toward dissociation of ordinarily con-elated char- 

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

 habit seen in 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 ah 1 sorts of structural variants and excesses which have 

 not yet had tune or, because of the passive part the animals play in their relations 

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

 definite scheme of correlation. 



The morphological difference between the pentacrinites and the coniatulids is 

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

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

 additional segments are formed, while hi the pcntacrinites rupture does not occur 

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

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

 the comatulids condense the entire stem within the compass of the first-formed 

 nodal. The morphological difference between the comatulids and the pentacrinites 

 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 ui differing from all other related types through discarding the proxi- 

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

 which hi the pentacrinites is indefinitely reduplicated. 



The genus TJiiottifricrinu-s occupies a position midway between them; species 

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

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

 be graphically expressed by the following formula: 



pentacrinites + comatulids 



2 = 1 tnouiencrinus. 



TliioUn ricrinus, however, is in the direct line representing 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 pcntacriiiites, Tlnolliericrmus and the comatulids repre- 

 sent a small group of which Tli'mU'it ricrin UN is the true phylogenctical exponent, the 

 other two types being aberrant departures from this stock. 



TTiioUcricrinus is fossil only. In the recent seas the comatulids far outnumber 

 all of the other crinoids taken together, at the same tune extending through a much 

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

 the remaining groups is that of the pentacrinitcs. 



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 



