194 



EEPTILIA 



The mandibular elements, which are six in number, are so intimately- 

 united in the adult as to appear like a single piece. A symphysial suture is 

 present in the Chelydridae, however, at least in young individuals. Teeth are 

 entirely wanting, both on the palate and jaw-bones, but the edges of the latter 

 are covered with sharp, horny sheaths like the beak of a bird. 



Limb-girdles. — The limb-girdles (Fig. 299) are remarkable for being enclosed 

 within the external shell, but as they precede the development of the anterior 



and posterior dorsal ribs 

 in the embryonic state, 

 their apparently abnormal 

 position is seen to be a 

 secondary modification. Of 

 the three branches which 

 constitute the pectoral 

 arch, namely, the scapula, 

 precoracoid, and coracoid, 

 the latter is longest in the 

 Chclonidae, the former in 

 all other Cryptodires. The 

 coracoids are long, distally 

 expanded bones, directed 

 backwards and inwards, 

 but not meeting in the 

 median line. The scapula 

 is slender, rod -like, and 

 upwardly directed. Its 

 distal extremity is attached 

 by ligaments or cartilage 

 to the anterior costal bone, 

 and its proximal end is 

 fused with the precoracoid, 

 which corresponds to an 



Cistudo lutarla, Marsili. [C.iEvm) europaea, Sclineid.] Recent; elongated acromial prOCeSS 



EuroiJe. Ventral aspect, the plastron removed to one side, c, Costal /,, i ;> Vi 



plates; co, Coracoid; c, Entoplastron ; ep, Epiplastron ; /, Fibula; fc, (prOSCapula, Jbaur; 



Femur ; h, Humerus ; hpp, Hypoplastron ; hyp, Hyoplastron ; jl, Ilium ; u ^l„,nVlp " Pn vipi- ^wpn^ 



;s, Ischium ; m, Marginals ; •»! it, Xuchal ; p6, Pubis ; j)sc, Precoracoid ; citivicit;, V-'UViei, v^wciiy. 



P!l, Suprapygal ; r, Radius ; sc, Scapula ; t. Tibia ; u, Ulna ; xp, Xiphi- Xhe latter is directed f Or- 

 plastron. 



wards and downwards, and 

 its expanded distal end is attached by ligaments to the entoplastron. The 

 humerus exhibits a large spherical head, and a more or less strongly curved 

 shaft. The radius and ulna are of about equal length, except in the Chelonidae, 

 where the former is much longer and situated below the ulna. The proximal 

 carpals are commonly four in number, and the distal five ; numerous varia- 

 tions, however, are exhibited by the bones of the manus amongst the different 

 families and genera. Five digits are always present, but sometimes not more 

 than three are clawed. 



In the pelvic arch the long ilium is loosely attached to the sacrum and 

 eighth costal plate in the Cnjidodira, to the sacral ribs in the Trionychoidea ; 

 but. in all existing Pleurodim the pelvis is solidly united with the carapace and 

 plastron, and bears no trace of sacral attachment (Fig. 307). The pubis and 

 ischium of Cryptodires form a ventral symphysis, and their symphysial branches 



