Logan.] The Invertebrates, Niobrara Group. 483 



and second radials, and the interradial plates, which are oppo- 

 site these. This arrangement is not constant. 



" The interaxillary plates vary also with regard to shape and 

 arrangement. In the axillary area of one specimen three pen- 

 tagonal plates are succeeded above by three pairs of smaller 

 plates of various shapes ; these followed, each, by two branches 

 composed of still smaller plates. In another axillary area of 

 the same specimen one pentagonal plate is succeeded above by 

 two heptagonal ones, which are in turn succeeded by three pairs 

 of smaller plates of variable shapes ; these each being followed 

 by two branches of smaller plates." 



Two large slabs of crinoidal limestone in the University col- 

 lection are almost wholly covered with Uintacrinus socialis. The 

 adhering forms on one of these slabs are evidently of adult size ; 

 on the other slab are a few individuals approaching adult size, 

 all the others being but one-fourth that size. It was thought 

 that two species are represented in these two slabs, but a care- 

 ful examination failed to reveal any specific differences except 

 in the matter of size. It is remarkable that there should be 

 such a uniformity of size among all the individuals of one slab. 



The limestone to which these two forms adhere is composed 

 almost entirely of the plates of this crinoid, the plates being 

 cemented together by a calcareous matrix. Nearly all of the 

 larger individuals present the lateral surface. This condition 

 is probably due to the stronger arms of the adult causing the 

 crinoid as it lay on the sea floor to be crushed laterally instead 

 of vertically. The fragile condition of the arms of the young 

 individual allowed it to be crushed vertically as it lay on its 

 ventral side. Specimens of Uintacrinus socialis are rare in the 

 Kansas fields. The specimens in the University collection were 

 collected from the lower part of the Hesperornis beds. 



