164 BRITISH FOSSIL REPTILES. 



is from one to two lines, and become gradually thinner to their peripheral border, 

 which, however, is too much fractured to show whether it has been terminated by a 

 dentated suture like the neural plates, which unite with the costal plates in the 

 ordinary Chelones. The degree of thinness of the actual margins of the large 

 scutes in question shows that they were not suturally united to costal plates. On 

 the hypothesis, therefore, that they are the median or neural plates of a Turtle, they 

 can only be referred, as not uniting laterally with costal plates, to the ninth and tenth 

 of the series of neural plates, which are under the same circumstances, and which also 

 differ from the eight preceding plates, in having contracted no osseous continuity or 

 adhesion to the subjacent neural spines. In order to test this particular conformation 

 I carefully excavated the chalk matrix beneath the median part of both scutes to 

 beyond the middle of it, and exposed only a smooth concave surface : there was no 

 trace of the median ridge, which is continuous with the summit of the spine, in the 

 first eight neural plates of the Chelonia. 



But besides the two plates, the exterior surface of which is exposed, there is a 

 third plate, the position of which is reversed, and which has slipped under one of the 

 scutes that has retained its natural position. A portion of a fourth similar plate is 

 also present in a similar reversed position in the same block of Chalk. This fact, 

 together with the thin borders of the plates, leads me to suspect that they may belong 

 to the series of marginal plates of a large Turtle, notwithstanding the open angle at 

 which the sides diverge from the median ridge, which, in that case, must have formed 

 the outer and anterior border of the carapace. 



On the hypothesis that these large plates have belonged to a Turtle, they indicate 

 an individual with a carapace between forty and fifty inches long ; as large, for 

 example, as that of which Camper makes mention in the memoir above quoted. 

 There is a possibility, however, that those large scutes may have belonged to some 

 Saurian reptile, although the probability is small, on account of the absence of any 

 rugosities, pits, or other sculptured character which marks the exteroir surface 

 of all the dermal bony scutes of Saurians hitherto found. It is possible that the 

 Poli/pti/chodon, or the Mosascmrus, if their skin was so defended, might have had 

 light and smooth scutes ; but the balance of evidence is at present in favour of the 

 Chelonian character of those in question. Their microscopic structure shows that 

 they have not belonged to a cartilaginous fish, and it agi'ees pretty closely with that 

 of the osseous tissue of unquestionably Chelonian neural plates of smaller size, from 

 the chalk formation. 



Another circumstance which also inclines me to view the large plates above 

 described as being Chelonian, is the corresponding thinness of the costal plates where 

 they are unattached to the subjacent ribs in the specimen from the Burliam Chalk- 

 pit, figured in PI. 46, fig. 3. The outer surface of these plates is also smooth, or at 

 most marked by fine strise. The borders by which they are in contact do not show 



