158 CHELONIA. 



48350. Slab showing the dorsal aspect of an imperfect humerus of 

 the same general type as the preceding specimens ; from 

 the Middle Purbeck of Durdlestone Bay, Dorsetshire. 



Beckles Collection. Purchased, 1877. 



43570. The proximal half of a left femur, agreeing in proportions 

 with the above humeri; from the Kimeridge Clay of 

 Weymouth. Purchased, 1871. 



Section III. PLEURODIRA. 



Shell with the pelvis suturally articulating or anchylosing to the 

 plastron ; usually covered with epidermal shields ; entoplastral oval 

 or rhomboidal, and not separating the epi- from the hyoplastral ; 

 a full series of marginals articulating with the ribs. A mesoplastral 

 present in some forms ; and an intergular shield developed in all 

 cases where epidermal shields occur. 



In the skull of existing forms (fig. 40) the quadratic articular 

 surface has a concavity for the reception of a condyle on the man- 

 dible ; the tympanic cavity is completely surrounded by the quadrate, 

 which thus forms an unbroken ring ; the pterygoids are very broad 

 and wing-like ; the vomer may be absent, so that the palatines may 

 meet in the middle line ; and there may be distinct nasals, and a 

 persistent suture in the mandibular symphysis. Cervical vertebrae 

 (when known) with well-developed transverse processes and single 

 termiual articulations ; sacral and caudal ribs articulating both with 

 arches and centra of vertebras. Not more than three phalangeals 

 in the digits. Head (at least in existing forms) retracted by a 

 lateral curvature of the neck. 



The labyrinth of the ear is always completely open from behind. 

 The ossification of the shell is generally complete (although there 

 may be persistent vacuities in the plastron), and the axillary and 

 inguinal peduncles of the plastron frequently extend a long distance 

 beneath the costals. In existing forms the neurals never form a con- 

 tinuous series with the suprapygals and pygal, and there is a great 

 tendency to a reduction in the number of the neurals ; in Jurassic 

 forms there may, however, be a full series of neurals in connection 

 with the suprapygals. The carapace is always suturally connected 

 with the plastron, and, except in the Triassic Proganochelys, there 

 are well-developed axillary and inguinal buttresses. Epidermal 

 shields are present on the shells of all known forms except the 

 Carettochelyidce. The humerus is always of the type shown in 



