BAENIDiE. 



6l 



shell was about 250 mm.; the width something over 200 mm. The sutures between the various 

 bones are not obliterated, but the preservation is such that they can not be satisfactorily traced. 

 Most of" the anterior border of the nuchal is broken away. The thickness at the free border 

 is 6 mm.; and this border was rounded in section. The third costal is much thickened for 

 the reception of the axillarv buttress of the plastron, and the fifth and sixth were similarly 

 thickened for the inguinal buttress. The second costal, near its proximal end, is 5 mm. thick. 

 The surface of the carapace presents evidences of a low ridge along the midline. On the 

 area of the first costal scute there is a low elongated boss; and just in front of it, near the border 

 of the shell, is a smaller one. In a complete shell there would probably be found a lateral 

 carina on each side. 



The first vertebral scute is small, having probably a length of less than 30 mm. and a 

 width of 60 mm. The second is 48 mm. long and 65 mm. wide; the third, 57 mm. long and 75 

 mm. wide. In the original description the figures indicating the widths of the second and the 

 third had exchanged places. The fourth was fully as long as the third. The first costal 

 scute is small, being about 36 mm. in fore-and-aft extent. 



The plastron lacks the hinder extremity (plate 8, fig. 1; text-fig. 36). The total length 

 must have been close to 205 mm. The breadth, measured on the mesoplastra and following 

 the curves, is 186 mm. The median region is slightly concave as far outward as a ridge which 

 joins the free border of the anterior lobe with that of the posterior lobe. From this ridge the 

 lower surface slopes upward and outward to the outer borders of the plastral bones. The 

 bridge has a fore-and-aft extent of 87 mm. The anterior lobe is short and narrow, the 

 length being 52 mm.; the width at the base, 72 mm.; at the hinder ends of the epiplastra, 38 

 mm. The latter bones are small, and thev meet along the midline, in front of the entoplastron, 

 only 5 mm. The entoplastron is relatively large, the length being 28 mm.; the width, 17 mm. 

 Seen from the upper surface, this bone is broadly spear-shaped, with an anteriorly directed 

 process, a longer one directed backward, and a right and a left process. Its length on this 

 upper surface is 33 mm. The free borders of the anterior lobe are rounded in section. The 

 thickness of its various bones is about 7 mm. 



On the upper surface of this plastron there is a low ridge passing from one axillary buttress 

 to the other, making the thickness of the bone at the midline 9 mm. A similar thickening of the 

 bones is found between the inguinal buttresses, the thickness becoming 1 1 mm. 



The mesoplastral sutures are distinct everywhere except near the midline in front of the 

 right mesoplastron. The left mesoplastron is 21 mm. wide at the midline and apparently 43 

 mm. at the outer end. The mesoplastron of the right side is only 36 mm. wide at the outer end. 



The posterior lobe is 83 mm. wide at the 

 base. It is flat below. On the upper surface 

 there is a thickening parallel with the free border 

 on each side. From the summit of the ridge thus 

 formed the surface slopes to the acute border 

 and toward the midline. Just behind the inguinal 

 notch the thickness of the bone is 14 mm.; where 

 the hypoxiphiplastral suture crosses the midline, 

 only4 mm. thick. 



The sulci are in general distinctly developt. Those behind the intergular are somewhat 

 obscure. The intergulars do not separate the gulars. The various scutes meet their fellows 

 along the midline as follows: Intergulars, 12 mm.; gulars, Q mm.; humerals, 32 mm.; 

 pectorals, 41 mm.; abdominals 27 mm.; femorals, 40 mm. The length of the anals is indeter- 

 minable. They lie partly on the hvpoplastral bones. On each bridge there are 3 iniramar- 

 ginals, whose outer borders rested on the bridge peripherals. 



The table herewith is intended to present the most obvious differences in the proportions 

 of the plastral bones in the three species, B. hatcheri, B. marshi, and B. callosa. The width of 

 the bridge is taken as the unit. 



It is seen that B. hatcheri has, relatively to the width of the bridge, large anterior and 

 posterior lobes; that B. marshi has both lobes small; and that B. callosa has the anterior 

 lobe short and of moderate width, while the hinder lobe is broad at the base. 



