I 102 



CLASS REPTILIA. 



full complement of suprapygal and neural bones is present, although 

 there are numerous exceptions. Excluding the Amphichelydian 

 forms, no known Cryptodiran has a mesoplastral bone, while in 

 the majority of cases, especially among existing types, the plas- 

 tron has no intergular shield 

 (fig. 1016). The entoplastral, 

 when present, is either ovoid, 

 rhomboidal (fig. 1016), or 

 -|--shaped (fig. 1009), and the 

 epiplastral joins the hyoplas- 

 tral. In the skull (fig. 1007) 

 the outer border of the tym- 

 panic cavity is always deeply 

 notched posteriorly ; the quad- 

 rate articulates with a con- 

 cavity in the mandible; and 

 the pterygoids are compara- 

 tively narrow and laterally 

 emarginate. There are never 

 more than three phalangeals 

 in the digits. The sacral and 

 caudal ribs (as in the Pleuro- 

 dira) articulate partly with the 

 centra and partly with the 

 arches of the vertebrae ; while 

 the cervical vertebrae have 

 very imperfectly developed 

 transverse processes. No ex- 

 isting Cryptodiran has distinct nasals, and at present there is no 

 decisive evidence of the presence of these bones in any fossil form. 

 The Cryptodira are further characterised by their power of retract- 

 ing the head by a sigmoid curvature of the neck in a vertical plane 

 directly within the carapace. This large section comprises the great 

 majority of the existing Testudinata, and at the present day is 

 mainly characteristic of the Northern Hemisphere, being totally 

 absent from Australia. The habits of existing Cryptodires may 

 be carnivorous or herbivorous, and either terrestrial, fluviatile, or 

 marine. 



Family Acichelyid^e. — Under this name may be grouped a 

 number of generalised Cryptodirans often spoken of as the Thalas- 

 semydes, and in some cases as the Eurysternidce. They occur typi- 

 cally in the Lower Kimeridgian Lithographic Limestones of the Con- 

 tinent, and are abundant in the Kimeridge Clay, while they survived 

 till the period of the Lower Greensand. They were probably 

 mainly of marine habits, since they appear to be very rare in the 



Fig. 1016. — Plastron of Kachuga tectum; from 

 India. One-half natural size, g, Gular shield ; 

 hum, Humeral do. ; pec, Pectoral do. ; ab, Abdo- 

 minal do. \fem, Femoral do. ; an, Anal do. ; e.p, 

 Epiplastral bone ; ent.p, Entoplastral do. ; hy.p, 

 Hyoplastral do. ; h.pp, Hypoplastral do. ; x.p, 

 Xiphiplastral do. 



