54 



FOSSIL TliRTI ES OF NORTH AMERICA. 



Glyptops pervicax sp. nov. 



Text-fig. 32. 



The type of the present species is No. 1018 of the American Museum of Natural History. 

 It was collected by Mi. Barnum Brown, in 1902, from near the base of the Graneros shales of 

 the Benton deposits, in Yellowstone County, Montana. These shales are of marine origin. 

 The locality more exactly described is on Brush Creek, 10 miles east of Pryor, in the county 

 named. The remains consist of the front and the dorsal region of the carapace and the greater 

 part of the plastron. The shell is that of an aged individual. Not a trace appears of the sutures, 

 so that the structures can not be in all respects accurately determined. 



The carapace had a length of about 375 mm. There was apparently a broad, low, rounded 

 rid<*e running along the back in the areas of the second, third and fourth vertebral scutes. 

 At the midline of the front, over the neck, there was a rounded excavation. The anterior free 

 border was obtuse and about 10 mm. thick. The sulci are obscure, due mostly to an adhering 



incrustation ot clay, but some of them can be traced. Portions of 

 the second, third and fourth vertebral scutes may be mapt out. 

 They were considerably broader than long. The third was 83 mm. 

 long and had a maximum width of 130 mm. 



On the visceral surface of the carapace may be seen the bases 



of the rib-heads. These were not so strongly developt as in G. 



plicatulus. In the latter, as shown in No. 336, A. M. N. H., the 



rib-heads have a diameter of 10 mm., while in G. pervicax the 



diameter is about 7 mm. in a larger individual. The whole series 



is shown, on one side at least. The extremity of the tenth rib 



was co-ossified with the eighth costal about 10 mm. behind the 



rib-head of the costal named. On the first costal plate there is a 



prominent ridge which ran from the first and second vertebral 



centra to the border of the third peripheral and met the buttress 



of the plastron. At the peripheral this ridge has a width of about 



18 mm. and a thickness of 11 mm. How high the buttress 



Fig. 32. Glyptops pervicax. asce nded can not be determined, for the suture is obliterated. 



Plastron ot type. X {. j t a pp ears that the buttresses and the costal ridges meeting them 



Shows scute areas. No. 1 j 1 .1 .1 n ,./ , 1 



s a m NT T-r were more strongly developt in this species than in (j. plicatulus. 



The total length of the plastron (fig. 32) was very close to 

 335 mm. The anterior lobe is broadly rounded in front, as it is in G. plicatulus. Its length 

 is 85 mm., its width about 150 mm. The free border is obtuse. The bone is about 10 mm. 

 thick. The limits of the entoplastron can not be determined. On the upper surface ot this 

 lobe, about 18 mm. behind the anterior border, there are two low processes, one on each side 

 of the midline, which were probably for ligamentous attachment of the procoracoid pro- 

 cesses of the scapula. In G. plicatulus the corresponding processes are about 34 mm. from 

 the border. 



The bridge has a width of 140 mm. Its inner end starts from a low ridge which runs from 

 the free border of the anterior lobe to that of the posterior. The outer ends of the bridges are 

 mostly missing. 



The length of the posterior lobe was approximately 1 10 mm.; the width at the base, 142 mm. 

 The free borders are acute on a level with the lower surface. From this edge the bone is beveled 

 and rises to a thickness of 12 mm. Beyond this, toward the midline, the thickness diminishes 

 again. At the hinder end of the lobe the thickness is only 5 mm. Most of the hinder 

 extremity of the lobe is missing. A small fragment is present, but on account of some doubts 

 it has not been used in the figure. On the upper surface of the lobe, very close to its border, 

 there is a circular depression about 7 mm. in diameter, which received a process of the ischium. 

 On the same surface about 60 mm. behind the inguinal notch, begins another depression, con- 

 siderably larger, for the reception of the pubis. 



Most of the plastral scutal areas can be mapt out. The intergulars measured 32 mm. 

 along the midline. Taken together, they had a width of about 28 mm. It can not be determined 

 certainly whether 01 not they were divided at the midline. The gulars, as in G. plicatulus, had 



