CRETACEOUS FORMATIONS. 13 



T. IV, fig. 1 is the slender portion of the entosternal, es, and a fragment of the 

 right hyosternal of a turtle, which must have been about one foot eight inches in length. 



Figure 2 gives an inside view of a rib, with the connate costal plate, the gradual 

 narrowing of which towards the free end of the rib resembles that in the Chelone 

 Benstedi. 



Figure 3 is a similar specimen from the carapace of a larger turtle, with the neck 

 of the rib more freely relieved from the connate costal plate. 



Figure 4 is a more mutilated example of a larger rib and costal plate. 



Figure 5 is the right hyposternal of the Chelone Benstedi, and has belonged to a 

 specimen not larger than either of those figured in T. I — III. 



Figure 6 is the humeral end of the connate scapula and clavicle of a turtle. 



Figure 7 is the outer side of a marginal scute of a large turtle. 



Figure 8 is the left humerus of a turtle, which differs from that of the existing 

 species in the greater expansion of its distal end. 



Figure 9 is the left ulna of a turtle, belonging to a larger example than that to 

 which the humerus belonged. 



I have been favoured with the opportunity of inspecting portions of the skeleton 

 of a large Chelonian obtained by Mrs. Smith, of Tonbridge Wells, from the lower 

 chalk at Burham, Kent, and skilfully relieved from their mineral bed by that lady. 

 The principal bones consist of two series, one containing five, the other three and 

 parts of two, of the marginal plates of the carapace, in natural connection, and from 

 that part of the margin where they receive the extremities of the vertebral ribs (T. VI, 

 figs. 1 and 2). These marginal plates in Chelone mydas are three-sided, and have two 

 thick terminal borders by which they are united, suturally, to one another : of the three 

 free surfaces, the one, directed towards the interior of the body, is concave and 

 characterised by a deep depression for the reception of the tqoth-like extremity of 

 the rib (fig. 2) ; the other two (upper and under) surfaces meet at an angle, which is 

 produced at certain parts to form the marginal dentations of the lateral and posterior 

 parts of the carapace in that species of turtle, but is more open and obtuse in the 

 marginal plates at the anterior part of the carapace. In the fossil the marginal plates 

 have the general characters of those of the genus Chelone, but differ from those of 

 the Chelone mydas in being more concave on the central or perforated side, and they 

 are also concave at the upper side, and in a slighter degree at the under side ; these 

 sides likewise meet at a more acute angle, and this angle is produced into a sharper 

 and more continuous ridge ; but this ridge subsides at one end of the series of plates 

 in fig. 1 , and the upper and under sides gradually meet at a more open angle, which 

 is rounded off in the first of the series. This plate, therefore, answers to the third 

 marginal plate in the Chelone mydas, or that which receives the end of the first 

 expanded vertebral rib ; and the remainder, therefore, to the fourth, fifth, sixth, and 

 seventh marginal plates : now these are precisely the ^marginal plates in the Emys 



