REPTILIA 



229 



clear up, for they arise from prlmordia internal to the ribs. The pec- 

 toral girdle consists of a triradiate group of flattened bones: the 

 scapula, the procoracoid and the coracoid, the last being the largest. 

 Together they 

 unite to form the 

 socket which re- 

 ceives the head of 

 the humerus. The 

 pelvic arch is 

 more compact and 

 is composed of the 

 pubis, ischium and 

 ilium, uniting to 

 form the aceta- 

 bulum for the 

 head of the femur. 

 The skull is 

 fairly generalized 

 in structure, but 

 has some special 

 features. The 

 jaws are devoid 

 of teeth, and max- 

 illary, premaxil- 

 lary and dentary 

 bones are covered 

 with hard chit- 

 inous sheaths, 

 that form the 

 upper and lower 

 members of the 

 cutting beak; the vomer is a single unpaired median bone; there are 

 no lachrymals nor ectopterygoids; the pterygoids send inwards wings 

 of bone, that, with the aid of the palatines, form a continuous roof 

 to the mouth; the supraoccipital is prolonged backwards into a 

 large narrow process upon which are inserted the heavy neck 

 muscles. All of these bones, even the quadrate, are firmly united 

 into a solid cranium. Further details of the skull are shown in the 

 figure (Fig. 131). 



Fig. 130. — Skeleton of tortoise, Cisludo lutaria', seen 

 from the ventral side with plastron removed and placed 

 to one side. C, costal plate; Co, coracoid; e, endoplastron, 

 ep, epiplastron (clavicle); F, fibula; Fe, femur; H, humerus; 

 Hyp, hyoplastron; Hpp, hypoplastron; II, ilium; Js, is- 

 chium; iW, marginal plates; A'^m, nuchal plates; Pb, pubis; 

 Pro, procoracoid process of scap>ila; Py, pygal plates; 

 R, radius; sc, scapula; T, tibia; U, ulna; Xp, xiphiplastron. 

 (From Parker and Haswell, after Zittel.) 



