TRIONYCHID/K. 



W5 



the ridge on the lower surface, is 16 mm. Where it made contact with the preneural the 

 thickness is only 4. mm. 



The portion of" the costal, the proximal halt", almost certainly belongs to the same indi- 

 vidual. At its proximal end it presents the sutural borders for the preneural and the first 

 neural. Evidently the preneural was somewhat wider than the neural. Its length was at least 

 40 mm. The border of the costal for union with the neural is 7 mm. thick. About the middle 

 of the length of the hinder border of the costal the thickness is 7 mm. 



The upper surface of the bone is incrusted with a layer of iron oxide ; but so far as can 

 be discovered the sculpture was finer than in the other two species found in that region, 

 ./. austerus and A. vorax. The few pits observed appear to be about 2.5 mm. in diameter. 



The respects in which this species differs from those just named are mentioned under 

 their respective descriptions. 



Aspideretes austerus sp. nov. 



Text-figs. 649, 650. 



The fragmentary specimen on which the present species is based was collected in 1904, 

 by Mr. Barnum Brown, from Laramie deposits at Ojo Alamo, San Juan County, New Mexico. 

 The catalog number of the specimen is 6068. The remains, belonging apparently to a single 

 individual, consist of the nuchal except the central portion, part of both first costals, the 

 greater part of a right posterior costal, probably the sixth, various fragments of costals, a 

 piece furnishing parts of two neurals, and a considerable portion of the right hvpoplastron. 



The species is characterized by the very thick bones and the coarse sculpture of both 

 the carapace and the plastron. 



Fig. 649. Aspideretes austerus. X I- Parts of nuchal and first costals of type. 



On account of the missing middle region of the nuchal, the exact lateral extent of this bone 

 can not be determined. It was, however, not far from 230 mm., being thus somewhat less than 

 that of A. fontanus, just described. The greatest width is 45 mm., a fourth less than that of A. 

 font anus. The greatest thickness of the bone is 21 mm. The free anterior border is not bev- 

 eled, but is cut off at nearly right angles with the upper surface, like that of the species just 

 mentioned. The thickness of this border varies from 10 mm. toward the midline to 15 mm. 

 near the outer end. 



The first costal is about 72 mm. wide near the neural border and it increases to 80 mm. 

 near the distal end. The free border is cut off at a nearly right angle to the upper surface. 

 The thickness of this border is 10 mm. or more. The hinder border of this bone is 10 mm. 

 thick. 



The posterior, probably the sixth, costal is 25 mm. wide 1 10 mm. from the distal end, but 

 it widens rapidly to 55 mm. The free border is like that of the first costal. There must have 

 been a considerable notch between this costal and the one next behind. In thickness this 

 costal ranges between 9 mm. near the proximal end and 10 mm. at the distal end. 



