Descriptions of New Crinoids. 253 



nature, or to represent an}^ excretor}' or other opening. Indeed, we 

 find authors searching the other extreme of the "ventral sac," of 

 which these plates are simply the lower extremity, and the presence 

 of which they invariably indicate, for such an orifice; and in most 

 cases unsuccessfuUj'. These plates, then, represent no functional 

 character, are but the exposed proximal part of the " ventral sac," and 

 can not have that systematic importance which belongs to radial por- 

 tions of the crinoids; and they are infinitely' less important than char- 

 acters found in the arras, which are entireh' neglected in the group- 

 ings of these crinoids made by the writers above mentioned. 



But, if the extent to which the parietes of the ventral sac are ex- 

 posed, by the relations between its proximal portion and the adjacent 

 plates is ever a generic character, it must alioays be so, no matter 

 whether the series of extremes is a long or a short one. In fact, we 

 have no means of testing the amount of difierentiatiou which animals 

 of a given class have undergone in a given time, but by referring this 

 differentiation to generic and specific standards, previously thoroughly 

 well established. 



If, then, the presence of one, two, or three az3'gos plates is to be re- 

 garded as a substantiated generic character, and if we find that within 

 a comparatively brief geological epoch, many crinoids have attained 

 this degree of diff"erentiation, we maj^ assume that it has been a period 

 of comparatively rapid evolution; but, on the other hand, if these char- 

 acters are not well defined, and if a proper stud}' of the specimens in- 

 dicates the mixed state of aff"airs, which their discussion of Eupachy- 

 criuus would lead us to infer, we can only understand that these cri- 

 noids ceased to exist, in the midst of a series of changes, always to be 

 observed, when we have an opportunity to investigate the phylogeny of 

 an}' group of animals. The three genera here discussed form a well-de- 

 fined group of crinoids, that might, without any violence to a proper 

 systematic arrangement, be placed in a family by themselves. It is 

 even plain that such an arrangement, including, possibly, a few nearly 

 related genera, would be an improvement upon that adopted by our 

 authors, and future studies, from more abundant material, will proba- 

 bly indicate the desirability if not the necessity of such a course. 



