HaV : TURTI.ES FROM I UK yUDIlII RiVEK BeDS. 



181 



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 upi)er surface is t:,;^ mm. The free borders of the anterior lobe 

 are rounded in section. The thickness of the 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 bone is found 

 between the inguinal buttresses, the thickness becoming 11 mm. 



The mesoplastral sutures (Fig. 2) are distinct everywhere, excej^t 

 near the midline in front of the right mesoplastron. The left mcso- 

 plastron is 21 mm. wide at the midline, and it appears to have had a 

 width of about 43 mm. at the outer end. The mesoi)lastron of the 

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



The hinder 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 rapidly to the acute free border and more gently toward the 

 midline. Just behind the inguinal notch the thickness of the bone is 

 14 mm.; where the hypo-xiphiplastral suture crosses the midline, 

 only 4 mm. thick. 



The sulci (Fig. 2) are usually distinctly developed. Those behind 

 the intergulars 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, 9 mm.; humerals, 32 mm.; 

 pectorals, 41 mm.; abdominals, 27 mm.; femorals, 40 mm. The 

 length of the anals is indeterminable. They lie partly on the hypo- 

 plastra. On each bridge there are three inframarginals, whose outer 

 borders rested on the bridge peripherals. 



The following table is intended to present the most obvious dif- 

 ferences in the proportions of the plastral bones in the three species, 

 B. hatcheri, B. inaisJii and B. callosa. The width of the bridge is 

 taken as the unit. 



