171 



exists in another extinct genus, the Pleurosternon, of the early Tertiary for- 

 mation of England, but in this it has the shape of a parallelogram.' 



The sternal bridges of Chisternon present four large scutal areas nearly 

 resembling those of Baetia arenosa. They are not quite symmetrical on the 

 two sides. 



The axillary scute area is pentagonal, and is the smallest of the series. 

 The anterior border is oblique, and bounds the axillary notch externally. 

 Two outer borders form an obtuse angle and join the third and fourth mar- 

 ginal areas. The inner border joins the pectoral area. 



The second submarginal area is second in size of the series. It is longer 

 than broad, and nearly quadrate, but has its outer angles cut oif. The inner 

 border conjoins the pectoral area; the outer the fourth and fifth marginal 

 areas. 



The third submarginal area is but little larger than the axillary area. It 

 joins the pectoral and abdominal areas internally, and the fifth and sixth 

 marginal areas externally. 



The inguinal area, the largest of the submarginal areas, is obliquely quad- 

 rate, longer than. broad, and with the outer angles cut off. The posterior 

 border bounds the inguinal notch ; the inner border joins the abdominal area, 

 and the outer border joins the sixth, seventh, and eighth marginal areas. 



The inferior surface of the plastron is comparatively smooth. Striatious 

 cross the sutures, and elsewhere it presents a finely reticulo-vascular appear- 

 ance. 



The fractured condition of the shell affords us an opportunity of seeing 

 the strong hyosternal and hypostcrnal columns which aid in sustaining the 

 carapace. These columns are broad, vertical plates reaching far into the cav- 

 ity of the shell and dividing it into three compartments, as in the Batagur of 

 India. 



The hyosternal columns are 2| inches wide from their inner concave bor- 

 der to the axilla. The aperture of the shell between them is a doorway 3 

 inches wide near the roof and 4^ inches near the floor. The hyosternal col- 

 umns, partially exposed in the specimen, appear to be co-extensive with the 

 anterior supports. 



The breadth of the shell of Chisternon undalmn, between the lateral 

 obtuse borders of the carapace, is 15 inches. The length of the shell, or of 

 the carapace, in a straight line is estimated to have been about a foot and a 



