DERMATEMYDIDJE. 245 



is broad and flat. It was truncated in front and broadly rounded behind. With only the 

 known materials it is impossible to state accurately the length and width of the anterior lobe. 

 The anterior, truncated end had a width of about 80 mm. The two epiplastrals joined along 

 the midline for about 37 mm. The anterior free border of the epiplastral is thick, but the 

 lateral border thins down backward to an edge. The size and form of the entoplastron can be 

 judged from the surrounding bones. Cope gives the length as 70 mm. and the width as 95 mm., 

 which dimensions appear to be correct. 



As will be seen from the plate 36, fig. 2, the sutures are extremely irregular in their course. 

 The entoplastral notch in the hyoplastrals is not symmetrical. On account of unsymmetrical 

 development, the right hypoplastron articulated with both hyoplastra, the left hypoplastron, 

 and with both xiphiplastra. The right hypoplastron is thus 27 mm. longer than the left. 

 These and other measurements are presented in the table on page 242. 



Professor Cope states that the length of the hinder lobe of the plastron is 155 mm.; the 

 width, 230 mm. He also says that a fragment of the bridge displays the axillary and part of 

 the anterior inframarginal scutes; but to the present writer this fragment seems rather to belong 

 to the inguinal region of the right side. The reason for this identification is found in the fact 

 that in A.punctaius and A.lacer the pectoro-abdominal sulcus starts off from the sulcus bound- 

 ing the axillary scute on the mesial side, while the abdomino-femoral sulcus does not come into 

 contact with the inguinal scute on the lower side of the plastron. In the fragment under con- 

 sideration the arrangement of the sulci conforms to that of the region of the inguinal notch. 

 On placing this fragment in this region the hinder lobe is found to have a length of 145 mm. 

 and a width of 230 mm., being unusually short in proportion to the width. 



The central portion of the hinder lobe is thick, but the free borders are thin and acute, the 

 thickness becoming gradually reduced. On the upper side of the xiphiplastra there is a 

 prominent curved ridge, and nearer the midline a deep depression. These mark the attach- 

 ment of the pubic bone. A rounded knob marks the position of the ischium. 



The fragment of the bridge which has already been referred to as belonging properly to 

 the inguinal region bears on its upper, or inner, surface a strong ridge which rises to form a 

 buttress against the carapace. The inner border of the buttress is sharp. There is another 

 fragment of the bridge present, the outer end of the right hyoplastral. This does not reach 

 so far forward as the axillary notch; backward it extends to the hyohypoplastral suture. 



The sulci of the plastron are mostly very distinct, but not broad nor deeply imprest. The 

 longitudinal median sulcus runs a meandering course, but anteriorly it is too obscure to be 

 traced. There was a large intergular, probably divided. Its width was about 40 mm. Its 

 hinder border encroacht slightly on the entoplastron. The gulars were widely separated by 

 the intergular. The pectorals met along the midline for about 42 mm. and overlapt slightly 

 the entoplastron. The abdominals occupied 135 mm. of the midline; the femorals, about 

 82 mm.; and the anals, also about 82 mm. The inframarginal scutes were large, the middle 

 one about 135 mm. long and 50 mm. broad. Outside of the inframarginals are portions of 

 3 marginals, the fifth, sixth, and seventh. The sixth is 137 mm. broad, running along the middle 

 inframarginal for the entire length of the latter. 



The surface of the plastron is finely granulated, vermiculated, or pitted, but no definite 

 pattern of sculpture can be made out. On each side of most of the sutures there is a band of 

 fine parallel grooves. 



Portions of 2 costals are present, showing the proximal ends. One of these is 55 mm. wide 

 at the proximal end and 8 mm. thick. On the inner surface there is seen the rudimentary 

 rib-head. The other costal, probably the left seventh, is 38 mm. wide proximally, but it widens 

 distally. It is notcht proximally for a neural, probably the sixth, and behind the latter it 

 apparently met the corresponding costal of the opposite side. The surfaces are smooth and 

 present no distinct evidences of sulci. 



The form of the plastron of this species is most like that of//, agilis, but it differs from that 

 of the latter in being relatively shorter, in having the lateral borders more convergent from the 

 base, and the bone does not thicken so rapidly from the acute free edges. The sculpture of the 

 plastron of A. syntheticus is quite different from that of. A. agilis, and the inframarginal 

 scutes are much broader. 



