5IO FRANK SPRINGER 



There is another modification, not suggested by anything apparent 

 in the primitive type, but affecting the general form and habitus of 

 these Crinoids in a way that is of considerable practical importance. 

 Anyone who has had occasion to arrange the fossil's of this group 

 cannot help being struck by the presence of two general types. One 

 is marked by a tendency of the calyx and arms to form a globose, 

 ovoid, or pyriform crown, in which the arms lie in close contact — 

 although in some genera the lower part of the rays are separated by 

 wide interbrachial areas, above which they come together again. In 

 the other, on the contrary, the tendency is toward a spreading crown, 

 caused by the increasing divergence of the rays upward. In the first 

 the plates of the rays and arms, and the intervening interbrachial 

 structures when present, are for a considerable distance up more or 

 less flush with one another exteriorly, so that the general curvature 

 of the crown is but little interrupted. In the second, the rays and 

 their divisions are rounded exteriorly, and the interbrachial spaces 

 relatively depressed, so as to emphasize the appearance of divergence 

 above alluded to. 



Between such forms as Ichthyocrinus or Lecanocrinus on the one 

 hand, and Taxocrinus or Onychocrinus on the other, there is not 

 the slightest difficulty in distinguishing by the above character. But 

 there are occasional species, otherwise characteristic of the first group 

 which are pretty deeply furrowed between the rays and arms, and 

 some of the second whose arms are habitually rather closely packed 

 together, which we could not so readily assign to their respective 

 groups, except for their evident connection with related genera whose 

 characteristic species fall within them without any trouble. On the 

 other hand, there are a few forms which we are inclined to transfer 

 from the group which they superficially resemble, because of some 

 peculiar association of other characters which indicate a probable 

 closer relationship elsewhere. 



Now, I confess myself unable to point out any satisfactory mor- 

 phological basis for the difference in habitus between these two divi- 

 sions, and I have much doubt as to its structural importance. Yet it 

 is so constant and well marked in many cases, and affords such a 

 palpable and convenient means of separation in this perplexing 

 group, that we find it of some use in our classifications. It formed 



