THALASSEMYDID^E. 



FIG. 155. Osteopfgis erosus. 



First and second peripherals and first costal of type. 

 Xj. First peripheral at the right. 



type specimen comprises all the peripherals of the left side, except the third, fourth, fifth, and 

 eleventh; the second, fourth, eleventh and half of the ninth of the right side; the first left 

 costal, lacking the distal end; the distal end of the first right costal, and many fragments of 

 other costals; two neurals and a portion of a suprapygal; some fragments of the plastron; 

 and two vertebrae. Cope mentions five vertebrae. 



An estimate, based on the dimensions of the peripherals, shows that the carapace was 



probably slightly shorter than that of the type of 

 0. gibbt; that is, it was about 740 mm. long in a 

 straight line. 



Professor Cope stated that this species is 

 abundant in the upper bed of Cretaceous green- 

 sand at several localities in New Jersey, but the 

 present writer knows of but one other specimen. 

 The nuchal bone is missing, but its form and 

 proportions were probably similar to those of the 

 nuchal of 0. emargmatus. The following table 

 presents the dimensions of the peripherals (plate 

 26, fig. 2; text fig. 155). The length is taken 

 along the free border. The height is the distance 

 at the anterior end of the bone, from the free 

 border to the costal border. The thickness is the 

 greatest taken at the anterior end of the periph- 

 eral. Where a definite inner face presents itself, 

 as from peripherals 4 to 8, the thickness is 

 replaced by the dimensions of these faces. The 

 dimensions of the hinder end of any bone are the same as the dimensions of the front end of 

 the succeeding bone. 



The free borders of the first (fig. 156) and second (fig. 157) peripherals are thick and 

 obtuse. On the fourth there is a subacute, slightly upturned free edge, which may be traced 

 forward as an obtuse keel, even to the first peripheral. On the fourth peripheral (fig. 158) 

 the upper and the lower faces form an angle with each other greater than a right angle; but on 

 the sixth (fig. 159) this is already less than a right angle; and more posteriorly the two faces 



become more nearly parallel (figs. 1 60, 

 161). The upper faces of the four ante- 

 rior peripherals are irregularly convex; 

 those of the sixth and seventh are some- 

 what concave; those of the others are 

 nearly plane. 



In the hinder end of the second 

 peripheral (fig. 157) is a deep pit for the 

 anterior prolongation of the hyoplas- 

 tron. Cope thought that this pit was 

 for the rib-end of the first costal. The 

 pits in the greater number of the periph- 

 erals are circular in section. Those of 

 the ninth, tenth, and eleventh are flat- 

 tened. In the lasfmentioned cases the pits produce notches in the upper faces of the periph- 

 erals. The anterior end of the inner face of the eighth peripheral is deeply excavated for the 

 posterior outer angle of the hypoplastron. The inner face of the seventh is considerably 

 excavated for the same purpose. Several shallow pits appear along the lower borders of the 

 inner face of the sixth and seventh peripherals for digitations of the plastron. There was no pit 

 in the eleventh peripheral. This bone had a thickness of 18 mm. where it joined the pygal. 

 As regards the connection of the peripherals with the costals, we appear to have the fol- 

 lowing: The first and second peripherals were closely sutured with the first costal. The distal 

 end of this costal shows that at least the anterior half of the third peripheral was sutured with 

 it. The anterior end of the upper border of the fourth costal is smooth; the posterior two- 



