EDENTATA OF THE SANTA CRUZ BEDS. Iiy 



The plates vary much in size and thickness in the different parts of the 

 carapace ; in the anterior region they are small and quite thick, posteriorly 

 they become larger and thinner, especially along the median dorsal line, 

 and reach a maximum size in the pelvic region, but in each transverse band 

 the plates grow smaller toward the lateral margins. 



As compared with the great Pleistocene genera, the plates of this cara- 

 pace are smooth and the sculpturing shallow. The pattern is that of a 

 large central figure, which in the anterior and lateral regions is oval and 

 in the large plates of the median and posterior regions becomes subcircu- 

 lar ; in most of the plates this central figure is flat, but in a varying num- 

 ber of the scutes in the posterior region it is distinctly convex and in 

 some specimens a small, rough tubercle arises in the center of the figure, 

 forming a partial transition to Cochhps. This large central figure is 

 slightly eccentric in position and is placed a little nearer to the hinder 

 margin ; it is bounded by an obscurely polygonal groove, from which 

 radiate other short grooves, marking out the small peripheral figures, of 

 which there is usually a single row on each plate and, owing to the eccen- 

 tric position of the large figure, the anterior peripherals are somewhat 

 larger than the posterior. Many scutes of the lateral region have a 

 more or less complete second row of anterior peripherals and thus, as 

 Lydekker has expressed it, two complete rings of polygonal peripherals, 

 "or a suturally divided single one may intervene between two centrals." 

 Small, pinhole like, piliferous pits are placed at the junctions of the main 

 groove with the radiating grooves, and at the intersections of the latter. 



The marginal scutes are considerably modified ; on the anterior border 

 the plates have no anterior peripherals and extremely small posterior 

 ones ; on the hinder border the posterior peripherals are wanting and the 

 central figure is drawn out into a blunt point, thus forming the serrate 

 hinder edge of the carapace ; on the lateral margin the plates have very 

 small inner peripherals, and the narrow, elongate central forms the actual 

 margin ; as each of these plates increases in width posteriorly, the hinder 

 portion of the plate projects below the level of the next succeeding one, 

 producing a moderate serration of the border. 



No complete specimen of the tail-sheath (Plate XVIII, figs. 2, 3) has 

 yet been obtained, but by comparing a number of imperfect individuals, 

 it may be fairly well reconstructed. The sheath consists of two quite dis- 

 tinct portions, an anterior portion, made up of quite freely movable, im- 



