498 Observations on the 



from the latter rocks, with his views concerning the pro- 

 priety of its establishment. These genera appear to be 

 founded, not upon detailed distinctions from Poteriocrinus, 

 as these authors found it constituted by Miller, but upon the 

 definiteness of the ensemble which a collection of the species 

 of each genus presented to the eye, with a constancy of form 

 and inflection in some of the parts of each. The peculiar ex- 

 ternal expressions of these genera were doubtless accom- 

 panied with as great, if not greater, modifications of inter- 

 nal structure, not now available in their classification ; but 

 they are scarcely less important if considered as only ex- 

 pressions of chronological characters in the mystic lan- 

 guage of organic forms. 



That there is great difference in the perspicuity and 

 practical value of generic characters is quite apparent ; but 

 we cannot admit that there is really a gradation from spe- 

 cific to generic and family characters ; and the actual ex- 

 istence of sub-genera may even be doubted, although our 

 present mode of classification seems to require their recog- 

 nition. 



The investigations of the past few years having resulted 

 in bringing to light so many nearly perfect forms of fossil 

 crinoids, which have added so much to our knowledge of 

 the various modifications of the structure of their superior 

 parts, and so plainly shown the impropriety of referring 

 such varied forms to the same genera, the time seems to 

 have arrived for a more full recognition of the generic 

 value of these modifications. I therefore propose for a 

 group of these fossils, of which Poteriocrinus dilatatus of 

 Hall is the type, the generic name of Cceliocrinus, to be 

 used in at least a sub-generic sense. 



