146 



THE EXTINCT BATEACHIA, REPTILIA 



mand record. Its form suggests the same portion of the Thalassochelys planimentum and Th. craasicostatus of 



Owen (Chelone Ow.) from the Eocene of England; but it is much more obtuse, and broader, in relation to its length, 

 than either of these. As the extent of the symphysis among Tostudinata bears some relation to the position of tho 

 posterior opening of tho narcs, the pertinence of this jaw to the Euclastes platyops suggests itself. Its flatness and 

 shallowness, and want of recurved alveolar margins, arc appropriate to the shallowness and small alveolar margin of 

 tho maxillo-palafine face. There is, however, no trace of that elevation and acummation of the extremity of the 

 symphysis, associated with the premaxillary concavity and foramen to tho nasal meatus, which is to be looked for, 

 and which occurs to some extent even where the foramen is wanting, e. g., in tho Thalassoohelys caouana. In its spe- 

 cific features, It is different from the E. platyops in its greater abbreviation. In the former, the width at 

 the anterior margins of the coronoid process is to the median length anterior to the line between these points, as 

 2:4-5; in the latter, measuring on the cranium below, as 2:1 3-5. 



In this species the posterior margin of the symphysis is opposite tho anterior part of the coronoid bono ; tha an- 

 terior margin of the ooronoid elevation of the dentale measures the posterior or third of the symphysis. The sutures of 

 the dentale with other elements nowhere consolidated. Tho internal groove passes round the hinder face of the arch, 

 situated deeply between the prolongations of the superior and inferior plates. Mandible beneath, flat, abruptly ris- 

 ing to tho alveolar ridge. This is broken away, but from fragments of surface remaining, has been but little elevated. 

 Superior alveolar faces, nearly plane, slightly elevated longitudinally on the median line, where tin; surface is slightly 

 striate. Tortious of the superficial plate remaining offer no evidence of an upward curvature, though a small portion 

 of the extremity has been worn off. 



A striking feature in this species is the size and depth of the fossa for the insertion of tho masseter muscle. It 

 is relatively greater than in any of the recent Cholonidio which I have seen, and indicates great muscular power. 

 This in cessitates the shortness of the mandible, as it is less strengthened in the vertical direction than the modern 

 species. The mental foramen enters in tho anterior portion of this fossa. The dental foramen, of equal size, enters 

 below the middle of the coronoid bone. The wall of the fossa is the inferior plate of the mentum prolonged into a 

 strong ridge. 



Measurements. 

 Length of symphysoal line, 

 "Width ai, anterior margins, coronoid processes, 



" apices coronoid bones, 

 Elevation apices coronoid hones, 

 Depth at symphysis behind, 



Lilies. 

 25 

 42.7 

 42.7 

 1!) 

 8 



Two portions of peculiar marginal bones, from the left side of the carapace of one of the Chcloniidae, found at 

 the same time and place as preceding, have much the: same mineral character, and probably belong to it. The piece 

 from the median margin is quite thick on the inner face, which, with the upper face, is concave; lower, slightly 

 convex. 



Lines. 

 Depth within, l0 - 5 



Width below, 15 - 5 



Width above, li5 



Fossa for costal extremity, open conic. 



A portion of what is perhaps a fourth left marginal, is triangular in section, each side concave, the inner fare 

 most extensive. The margin is obtuse, and presents an open concavity, thus connecting the forms of the second and 

 fifth bones here described. 



Width of inferior margin, 9 - 5 Hn ™- 



Second marginal very shallow, vertically; a very open cmargination between dermal shields, which have left a 

 shallow sutural groove. It presents a narrow truncate inner margin, and there is no rib-pit in it. 



