NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 77 



the Bourgueticrinidse for the ancestors of this genus, though not nec- 

 essarily by way of either Mesocrinus or ThioUiericrinus. It may well 

 be that fusion of columnals mtli the proximale is a character pro- 

 gressively acquired in a manner comparable to that in which we note 

 the appearance of cirri on the centro-dorsal. Fusion of additional 

 columnals in such a case would then merely indicate a comparatively 

 high degree of specialization. 



Polypliyletic nature of the Comatulse. — As has been elsewhere sug- 

 gested, it is highly probable that the Comatulse represent terminal 

 members of a number of lines of development in each of which an 

 eleutherozoic existence has been independently acquhed. They 

 may all be considered as offshoots of the same general stock, and con- 

 vergence has served to evolve strikingly uniform types. Again, the 

 different lines have had their inception at mdely different time peri- 

 ods as is evidenced by the extremely variable degree of specialization 

 exhibited by contemporaneous forms. It is probable that we must 

 look among the Apiocrinidne as defined b}' Bather (1900) as well as 

 among the Bourgueticrinidse for the ancestoi"s of the Comatulse. 

 Again, it is quite witlim the bounds of possibility that the Pentacri- 

 nidse may have given rise to such eleutherozoic forms. 



The structural characters that seem to indicate the presence of a 

 number of genetic hnes among the Comatulse may not here be gone 

 into in any considerable detail. The evidence on the whole is sug- 

 gestive rather than conclusive and will oid}^ become of positive value 

 when the several lines are worked out and their mutual relationsliips 

 established. The nature of the evidence is of two sorts. The first is 

 as regards the relative speciahzation of contemporaneous types as 

 indicating the relative remoteness from stalked ancestors. The sec- 

 ond relates to the structure of the organisms, as indicating the mutual 

 interrelationships of the forms. 



The disparity in degree of specialization among the eleutherozoic 

 forms at any given time may, if judiciously considered, be given con- 

 siderable weight. Thus in the Cretaceous TMolUericrinus we may 

 well have the point of inception for a comatuhd line. Living at the 

 same time were higlily specialized Comatulse, and these were preceded 

 even as far back as early Jurassic time by equally distinctive eleuthero- 

 zoic types. Concerning the high degree of development to be observed 

 in these early fosssil forms Carpenter (1884) writes as follows: 



Besides their tendency to combine the characters of recent generic types, the 

 Jurassic Comatulae are remarkable for their large size, as are also the Cretaceous spe- 

 cies. The centro-dorsal may reach from 9 to 13 mm. in diameter, which is greater than 

 that of nearly every recent species except Antedon eschrichti; while this type and 

 Actinometra robusta are almost the only living Comatulse with arm-bases anything 

 like as massive as those of the fossil species. Some of the Cretaceous forms must have 

 been very large. Thus the united centro-dorsal and radials of Antedon campichei from 

 the Neocomian of Switzerland may reach 15 mm. in height and over 20 mm. wide; 



