THALASSEMYDIDJE. 



The humerus (fig. 163, hum) has an extreme length of 170 mm. The shaft dorso-ventrally 

 is 17 mm. thick. The smooth rounded end is 54 mm. wide. The ulnar crest rises above the 

 head. Between the plane of the two proximal processes the angle is obtuse. 

 The pelvis (fig. 163 II, isch, pub) resembles in general that of Chelydra. 

 The specimen referred to above as having been described by Cope as Osteopygis sopitus 

 is now in the American Museum of Natural History and has the number 2351. It consists 

 of peripherals one to four, seven, eight, and ten, of the right side; peripherals one, three to 

 five, seven, eight, and ten, of the left; the nuchal, the proximal portions of costals one to 

 four of the left side; one nearly complete costal, perhaps the third of the right side; many 

 fragments of other costals; a number of plastral bones; parts of both humeri; and a complete 

 left femur. 



Cope has, from this specimen, produced a figured restoration of the carapace (Ext. Batr. 

 Rept., etc., p. 139, fig. 39). This is represented as having 10 pairs of costal plates and a corre- 

 sponding number of neural bones. It is difficult to understand how that author could suppose 

 that the rib of the first costal could extend far enough forward to enter the pit in the second 

 peripheral. Evidently he saw no other possible use for the pit. The eleventh peripheral is 

 not present. On the tenth is found the number "10," written in ink; but at that time the 

 right hand digit "o" was written upon a "i." Altho on page 235 of the work cited, Cope 

 concluded that the genus Osteopygis had only nine pairs of costals, he maintained, from exami- 

 nation of the present specimen, that Propleura had ten pairs. Nevertheless, in his Vertebrata 

 of the Tertiary, publisht in 1884, he stated that Osteopygis had 10 pairs and Propleura 9 pairs. 



Cope figured a part of the nuchal, 

 the first left peripheral, and a part of 

 the first left costal. They are refigured 

 (fig. 169, p. 145), with the addition of 

 fragments overlookt by Cope. The 

 front of the carapace, along the nuchal, 

 was somewhat concave. The free bor- 

 ders of the nuchal and the anterior per- 

 ipherals are obtuse and thick. At the 

 midline the nuchal is 10 mm. thick. 

 The fore-and-aft length of the nuchal 

 is 78 mm. ; its width along the free bor- 

 der was close to 120 mm. The border 

 which joined the first neural is only 5 

 mm. thick. The table gives the dimensions of the peripherals, the measurements, except 

 the lengths, being taken at the anterior ends. In a few cases a dimension has been taken 

 from the contiguous end of the peripheral in front. 



The first peripheral was suturally joined to the first costal; probably also the second, but 

 the border is missing. It is probable that all the other peripherals to the eleventh were free 

 from the contiguous costals, but the upper border of the fifth has some appearance of having 

 formed a suture. The upper faces of the anterior peripherals are nearly plane. Beginning with 

 the fifth, the upper faces are more or less concave from the free edge to the costal border. The 

 hinder peripherals have a thin acute border and this may be traced forward, but becoming less 

 acute, to the first peripheral. In the hinder end of the second peripheral, in the inner face, is a 

 pit, like a deep thumb impression, for the anterior prolongation of the hyoplastron. The other 

 peripherals have each a pit for the end of the corresponding rib. They are at the middle of the 

 length of the inner face. Most of them are circular in section, but those of the last two or three 

 peripherals are somewhat flattened; and they notch the upper face of the bone. 



The first costal has a maximum width of 98 mm. The second is 66 mm. wide; the third, 

 60 mm.; the fourth, 46 mm., all measured at the costo-vertebral sulcus. At the sutural 

 borders these bones are 5 mm. thick. 



The surface of most of the bones is more or less roughened; and on many of them, espe- 

 cially the costals, there are present scattered pits, each about 3 mm. in diameter. These pits 

 are less conspicuous on the hinder peripherals. 



