PI,ASTOMENIDj<E. 



475 



FIG. 631. Plastomenus thomasi. Carapace. X J. 

 No. 6018 A. M. N. H. 



The bones of the plastron which Professor Cope was disposed to take as the type of P. 

 thomasi are now in the American Museum of Natural History, and are described under 

 P I 'at y pelt is serialis. 



Those specimens which Professor Cope figured in his great work of 1884 under the name 

 P. multifoveatus must be regarded as the type of the present species. Of these specimens 



that which furnisht fig. 6 of plate xviii is 

 now in the American Museum of Natural 

 History. The others are in the U. S. 

 National Museum at Washington. Of 

 these figures, 2 and 5 represent the distal 

 ends of 2 costals; fig. 3, the outer end ot 

 the right hyoplastron; fig. 4 is the left 

 hypoplastron; and fig. 7, the right hypo- 

 plastron. Figs. 2 and 5 seem to indicate 

 that the sculpture is irregular, but in the 

 text Cope states that the ribs intervening 

 between the pits tend to connect into ridges 

 running diagonally across the costal bones. 

 These specimens were obtained on Cotton- 

 wood Creek. 



The American Museum party of 1903 

 operating in the Badlands of the Bridger 

 Basin discovered a nearly complete shell 

 which is identified as belonging to Cope's 

 P. thomasi. It was found at Grizzly 

 Buttes, and the catalog number of the 

 specimen is 6018. The carapace lacks only 

 unimportant fragments. From the plas- 

 tron there are missing the entoplastron, the epiplastra, the median end of the right hyo- 

 plastron, and the right xiphiplastron. 



The carapace (fig. 631) has a length of 218 mm. and a breadth of 210 mm. The upper 

 surface was evidently moderately convex during life. The nuchal bone extends from side to 

 side a distance of 97 mm. and from front to back 30 mm. There are present a preneural and 



six neurals. It is probable that there was a small seventh 

 neural in a notch between the costals of the seventh pair. 

 The table shows the dimensions of these elements. 



The costals of the eighth pair meet along the midline 

 48 mm. and each measures 48 mm. along the free border. 

 The outer ends of the ribs extend but little beyond the bor- 

 der of the disk. The outer ends of the costals are about 7 

 mm. thick; and between the upper and lower layers of the 

 bone there is a channel running around most of the border. 

 On each side of the upper surface there are seen about 

 8 low and broad welts; 2 or 3 of these extend on the nuchal. 



The nuchal and the outer ends of the costals are ornamented with shallow pits. These are 

 mostly arranged in rows parallel with the border of the disk. A line 10 mm. long extends 

 across 4, sometimes 5 pits. On the neurals and the proximal ends of the costals the pits are 

 nearly obsolete. 



The form of the plastron and the proportions of its different parts may be seen in fig. 632. 

 The width of the plastron between the ends of the hyohypoplastral suture is 210 mm.; the 

 length along the midline, excluding the missing elements, is 135 mm. The bones of the 

 opposite sides are joined by jagged sutures. The hyohypoplastral suture of each side makes 

 an angle of about 75 with the median longitudinal suture. The greatest width of the hyo- 

 plastron is 47 mm.; the width of the midline, 25 mm. The width of the hypoplastron at the 

 midline is 49 mm.; of the xiphiplastron, 60 mm. The width of the hinder lobe, at its base, is 

 100 mm.; its length is 85 mm. The thickness of the hyoplastron at the midline is 5 mm.; 



