198 



THE DKSRADO FORMATION OF PATAGONIA 



the generic names of the Santa Cruz for their description, 

 and, so far as known, they are little differentiated from 

 those genera. There is as yet no material which shows the 

 association of skeletal parts with the carapaces. There- 

 fore, in this paper, comparisons are made wholly on the 

 carapace, with the expectation that the skeleton, when 

 found, will correspond. 



The Deseado species are but little less specialized than 

 the Santa Cruz, the carapace consisting of movable over- 

 lapping bands of plates both in the anterior and body 

 portions, while over the pelvic region the plates are fixed, 

 do not overlap, and form a pelvic buckler. 



Ameghino has described a considerable number of genera 

 based on isolated plates, to which I refer later. The 

 chief genera which occur in these beds are also found in the 

 Santa Cruz, and the distinguishing features are as follows: 



Proeutatus Ameghino 



Eutatus Amegh., in part, 1887, Bol. Mus. La Plata, t. I, p. 25. 

 Proeutatus Amegh., 1891, Revista Argen. d. Hist. Nat., t. I, p. 327. 

 Thoracothcrium Mercetat, 1891, Revista Mus. La Plata, t. 2, p. 42. 

 Eutatus Lydekker, in part, 1894, Anal. Mus. La Plata, t. 3, p. 62. 

 Proeutatus Scott, 1903-5, Reports Princeton Patagonian Exp., vol. 5, p. 40. 



This is the most frequently occurring genus in the 

 Deseado, but is as yet represented only by isolated plates. 



