10 UIXTACRINUS: ITS STEUCTUEE AND EELATIOXS. 



bers. The gigantic Haplmcapha, whose shells are sometimes over four feet 

 across, is almost equally prevalent. The sea must have swarmed with 

 Mososauroid reptiles, of which upwards of two thousand specimens have 

 been obtained in the region under consideration. But isolated specimens 

 of Uintacrinus have not yet been found there ; and in view of the excep- 

 tional opportunities presented for discovering the fossils which exist, it is 

 fair to conclude that, as, a rule, it did not occur in that way. 



The colony of Locality No. 2 were all small ones ; not a single specimen 

 of a size at all comparable with most of those of the other localities was 

 found there. Among the great numbers found at Locality No. 1, only 

 a very few young individuals were observed, — probably not over a dozen 

 in all, — and these were larger than the average of those at No. 2. 



While on morphological grounds I see no reason to doubt tliat all the 

 Uintacrimis thus far found in the Niobrara Chalk belong to one species, 

 there is a special reason, based upon their evident gregarious habit and 

 mode of occurrence, for believing that all the individuals of each colony 

 belonged to the same species. 



This would be in conformity with the known facts regarding the 

 living Crinoids, both stalked and unstalked. 



The stalked Crinoids of the present seas seem not only of wide geo- 

 graphical distribution for certain species, but also to be very gregarious in 

 their mode of life. Mr. A. Agassiz, in his account of the dredgings of the 

 " Blake " in the West Indies,* says of Pentacrinus : " We found them at 

 Montserrat, St. Vincent, Grenada, Guadaloupe, and Barbadoes, in several 

 places, in such numbers that on one occasion we brought up no less than 

 one hundred and twenty-four at a single haul of the bar and tangles. We 

 must, of course, have swept over actual forests of Pentacrini crowded 

 together much as we find the fossil Pentacrini on slabs." Rhizocrinus is 

 equally plentiful in West Indian waters, where Agassiz saysf that on one 

 occasion off the Florida Reefs " we must have passed over a field of 

 Rhizocrinus with the dredge, judging from the number of stems and heads 

 of all sizes which it contained." , 



The tendency to live together in swarms is equally characteristic of the 

 unstalked Crinoids. The free floating Comatulos also occur at times in 

 colonies, and must float or swim about in vast numbers. Agassiz {O^}- 



* Bull. Mus. Conip. Zool., 1879, Vol. V., p. 296. 



t Three Cruises of the " Blake," Vol. II., Bull. Mus. Comp. Zool., 1888, Vol. XV., p. 118. 



