THALASSEMYDIDJE. 



H7 



this as a mistake. After passing about 6 mm. the sulcus between the nuchal scute and the 

 hist marginal, the sulcus between the nuchal and the vertebral scutes suddenly becomes very 

 shallow, but the writer believes that it continues on in its usual position. 



\ considerable part of the suprapygal (fig. 173) is missing. Cope's statement that it is 

 only 2 inches and 4 lines wide is an error or meant to apply to some other bone. The bone in 

 question is at least 1 10 mm. wide. It occupies an area that in O. gibbi is occupied by two bones, 



but a close examination shows a line running 

 from the upper angle of one eleventh peripheral 

 to that of the opposite side, along which 2 bones 

 have co-ossified. The angulation of the upper 

 border of the eleventh peripheral shows that 

 there were two suprapygals. It is probable that 

 the sutural edge of the eleventh peripheral joined 

 the anterior suprapygal instead of the eighth 

 costal plate. The dimensions of the peripherals 

 are shown in the table. 



The sixth peripheral (figs. 174, 175) presents 

 three faces, an upper concave, a lower convex, 

 and an inner irregular. The upper and lower 

 faces meet at the acute free border. In the 

 inner face is a large, somewhat flattened pit for the end of a rib. Its mouth occupies one- 

 half the length of the face. Along the lower border of the face are two or three shallow pits 

 tor digitations of the hypoplastron. 



As we proceed forward from the sixth peripheral the free border becomes less acute, 

 until, on the third, it is obtuse and the upper, now convex, face rounds into the lower. On the 

 second and first peripherals (figs. 176, 177) the lower face becomes the obtuse free border 



of the bones. The inner faces of 

 the fourth and the third contain pits 

 for the corresponding rib-ends. The 

 inner face of the second has a large 

 excavation which received the anterior 

 outer process of thehyoplastron. Fig. 

 176 represents the first, second, and 

 third peripherals of the right side. 

 The upper border of the first and 

 the anterior half of that of the second 

 peripheral had a sutural articulation 

 with the first costal (fig. 176). The 

 thickness of these borders is 6 mm. 

 The hinder half of the upper border 

 of the second and the upper borders of all the other peripherals to the eleventh are smooth. 

 The upper border of the eleventh appears to have articulated with the suprapygals, as 

 already stated. 



Passing backward from the sixth peripheral, the upper and lower borders of all the 

 peripherals broaden and become flatter. The inner face narrows, and finally on the eighth 

 1 figs. 178, 179) and succeeding peripherals, curves into the lower face. Each of these periph- 

 erals, except the eleventh, has a pit for a rib-end. The hindermost peripherals are thin, the 

 ninth being 14 mm. thick, the eleventh about 10 mm. The flattening of the rib-pits begins with 

 the sixth peripheral. The pygal is represented by fig. 173. Fig. 180 shows one lateral border. 

 There are present manv fragments o( costal bones, but no complete costal. The remains 

 seem to show that the carapace was rather flat. The thickness of the costals near the neurals 

 was about 6 mm.; near the distal ends, about 5 mm. The rib-heads were strongly developt. 

 Distally, each rib projected beyond the costal and entered the pit in a corresponding periph- 

 eral. The edges of the costal plates approacht closely the upper borders of the peripherals. 

 On the proximal ends of two costals the costo-vertebral sulci run along about 35 mm. from 

 the neural border. Another fragment shows that the vertebral scute was strongly angulated 



Fig. 174. Osteopygis platylomus. Sixth left periphera 

 of type. xi. 



a, from above; b, hinder end; c, anterior end. 



