518 FOSSIL TURTLES OF NORTH AMERICA. 



The nuchal is now 280 mm. long from side to side, and it was originally perhaps nearly 

 300 mm. The fore-and-aft measurement in the midline is 45 mm. It narrows toward the outer 

 ends. The anterior margin is nearly straight, and the bone is 12 mm. thick in most of its parts. 

 The anterior portion of the upper surface is occupied by a smooth band running from the one 

 extremity to the other. The outer ends of the costal plates are ornamented with pits and 

 furrows varying from 2 mm. in diameter near the free border to 5 mm. away from the border. 

 These are arranged mostly in rows parallel with the free border of the costal. Elsewhere the 

 pits are irregularly distributed. 



In 1902, Dr. W. D. Matthew and Mr. Walter Granger, employed by the U. S. Geological 

 Survey, collected at Opal, Wyoming, two nearly complete carapaces of a trionychid which is 

 without doubt identical with Cope's Bridger specimen. These deposits at Opal belong to the 

 lowest of the Bridger formation. Fig. I, plate 99, shows the form of the better specimen. A 

 study of these and a specimen obtained for the American Museum of Natural History by 

 Messrs. Granger and Sinclair, in 1905, have convinct the writer that it is better to describe 

 them under a new name than to follow Cope's example of assigning them to A. radula, a 

 species from the Wasatch. 



This was an exceedingly large turtle, the length of the better carapace from Opal (plate 

 99, fig. i; text-fig. 672), along the midline, being 431 mm. and the width, greatest slightly 

 behind the middle of the length, 400 mm., not including the extension of the ribs beyond 

 the border of the shell. The carapace was only slightly convex, with a slight longitudinal 

 depression in the central portions. Anteriorly and posteriorly the carapace is truncated, with 

 the result of making it appear nearly square. The anterior border of the nuchal bone is 

 especially straight. Its size and appearance is almost identical with that of Cope's Bridger 

 specimen and needs not to be described. Its distal ends lie above the contiguous parts of the 

 first costals; and there are no fontanels behind the nuchal. There are eight neural plates, 

 there being one which appears to belong to the eighth pair of neurals, an unusual thing. Of 

 these, the first is the largest and the eighth the smallest. A view of the figure will show that 

 there exist various irregularities in the forms of the neurals and costals. The first neural has 

 a length of 70 mm. and a width of 55 mm.; but the second specimen has a smaller neural, 

 the anterior end of which is not so broad as the posterior. The third neural of the large speci- 

 men is 50 mm. long and 39 mm. wide. 



It will be observed that the seventh neural is heart-shaped and is brought forward so as to 

 lie partly between the costals of the sixth pair. The eighth neural is very small, 12 mm. long 

 and 19 mm. wide, and lies between the seventh costals. The eighth pair of costals measure 

 fore and aft, 44 mm. and from side to side, taken together, 136 mm. As an individual 

 peculiarity, the right one is much the larger. 



The borders of the shell are beveled off obliquely at the sides, and nowhere does the super- 

 ficial layer overhang the ribs. Posteriorly the edge of the shell is rounded. The ribs project 

 beyond the shell about 30 mm., and these projecting ends are 25 mm. or more wide. At their 

 sutural borders the costal plates are about 8 mm. thick. 



As stated, there is a smooth band along the front of the nuchal bone. In the central 

 portions of the shell (plate 99, fig. 2) the ornamentation consists of large shallow pits sur- 

 rounded with a low narrow wall. Here the pits average about 5 mm. in diameter. As the 

 outer ends of the costals (same plate, fig. 3) are approacht, the pits become slightly smaller 

 and are arranged more or less in rows which are parallel with the free border of the costal. 

 There may be as many as six pits in a line 20 mm. long. Here also the walls show a tendency 

 to break up into short ridges and tubercles. At the end of the plates there is a smooth band 

 from 1 8 to 25 mm. wide. 



The vertebral column is strongly developt, as shown by fig. 673. 



The second specimen, from Opal, is greatly like the one just described; but the greatest 

 width is slightly further forward and the hinder border is not so squarely truncated. 



The specimen collected by Messrs. Granger and Sinclair at the mouth of Big Sandy 

 Creek, Wyoming, No. 3938 of the American Museum, resembles in essential particulars the 

 carapaces just described, but it appears even flatter than these. The length is 450 mm., the 

 breadth, 370 mm. The ribs projected beyond the free margins at least 30 mm. There is 

 a broad smooth band around the border, except in the rear. The ridges surrounding 



