A MONOGRAPH OF THE EXISTING CRINOIDS 57 



arms have given rise to the recent Mariametrida. This group is more specialized 

 than the solanocrinids in the appearance and arrangement of the cirrus sockets, the 

 far advanced reduction of the basals, the development of the calcareous plug, the 

 numerous arms, and the many syzygial septa. The relatively stout arms, the small 

 centrodorsal cavity, and probably the relatively few syzygies may be considered as 

 primitive characters that have been retained. In addition to these, the Zygometridae, 

 Mariametridae, and certain Himerometridae show a number of primitive features. 



The third group, which possibly has a double origin, is represented by the Noto- 

 crinidae and Conometridae. It is divided into two distinct series of which one, the 

 Notocrinidae, leads toward the Palaeantedonidae and group a of the Macrophreata, 

 while the other leads to the Thalassometrida and group b of the Macrophreata. A 

 form that possibly may be interpreted as a prototype of one or possibly of both of 

 these series is found in Pterocoma pennata. The Thalassometrida have preserved a 

 number of pentacrinid characters in the long cirri, somewhat prismatic in cross section 

 and arranged in radial columns, in the prismatic or triangular brachials and pinnule 

 segments, in the relatively well developed basals, in the broad free dorsal surface of 

 the radials, in the strong development of the side plates and covering plates, and in 

 the granular covering of the disk. 



The nearer one comes to the present time the more difficult it may become in 

 critical cases to distinguish the types of these different groups from each other. If 

 one excepts the comasterids, which have followed their own special course of develop- 

 ment, and certain Mariametrida in which the reduction of the radial muscular fossae 

 is strongly pronounced, the comatulid groups tend toward an ideal type of comatulid. 

 For this reason it becomes extremely difficult to keep apart the converging forms that 

 by different paths have approached this ideal type. 



Gislen considered the subfamily Perometrinae as representing such a series, lead- 

 ing from colobometrid forms toward the antedonid type. He said it might have been 

 desirable to divide the Macrophreata into more groups that could later have been 

 placed at the apices of the different series of the older comatulid types. He did not 

 choose to do this, however, as it seemed to him to be systematically impracticable 

 and unfeasible. So he contented himself with depicting instead as nearly as possible, 

 guided by the known facts, the different paths of development through which the 

 distinctly emphasized polyphyletic character of the macrophreate group becomes 

 explicable. 



NOTES 



Dr. Gislen regards the approach toward an idealized — perhaps more properly 

 speaking generalized — type of comatulid that is evident in the several groups as con- 

 vergence from originally distinct lines of different ultimate origin. To the author it 

 seems more logical to reverse the picture and to assume a gradual divergence of the 

 several groups from a number of types originally very similar and of an ultimate 

 common origin. The comatulids have too many fundamental features hi common 

 that are not shared with other groups, even with the pentacrinites, to be considered 

 polyphyletic. 



From his generalizations Gisl6n omits the Comasteridae, which he said have 

 followed their own particular course of development. He did not mention the genus 



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