654 1'IIOCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



the comasterid species is necessary on account of the habits of those 

 animals, for a large amount of inorganic matter is ingested with the 

 food, necessitating a large absorption surface; the comasterids of 

 deep water have short digestive tubes and central mouths, like the 

 species of other families. Uintacrinus, being a pelagic form, probably 

 lived at or near the surface, and therefore its food consisted largely 

 of minute plants. This would be sufficient to induce a very con- 

 siderable increase in the length of the digestive tract over that 

 necessary for the assimilation of purely animal nutriment. Marsup- 

 ites had a central mouth; we can not say that it did not have a long 

 digestive tube; if it had a short one, it may have been strongly 

 located like that of some of the recent endocyclic forms. 



The position of the mouth is of no particular importance, system- 

 atically, in regard to the question of the relationships of these genera. 

 Among the Comasteridse most of the species have a marginal or a 

 submarginal mouth, though a number have it perfectly central as in 

 Antedon, as, for instance, Comatilia. Moreover, in those comasterids 

 in which the mouth is marginal it is always central in the young, 

 and does not begin to move from its central location until the orals 

 have become entirely resorbed. Thus in Marsupites, with its large 

 orals, we should not expect to find the mouth eccentric, no matter 

 what might be its position in nearly related genera. 



The crinoids are primarily fixed types inhabiting shallow water, 

 derived, not remotely, from free living littoral bottom inhabiting 

 animals. A pelagic crinoid, or a crinoid in the deep sea, is a crinoid 

 living under conditions not normal for its class, and in such a crinoid 

 we must always be prepared to find some structural inconformity 

 which, unless we are careful, will prevent us from appreciating its 

 true affinities. 



