THE EXDOSKELETOX 



719 



now being" greatly simplified. Most species of turtles to-day show a cer- 

 tain percentage of individuals with supernumerary scutes and plates. 

 (Fig. 424.) 



Fig. 424. Various plastra and their horny shields. 



1, Testudo ibera; 2, Macroclemmys temmincki ; 3, Cinosternum odoratum; 

 4, Sternothaerus nigricans; 5, Chelodina longicollis ; 6, Chelone mydas. a or 

 an, anal shield ; abd, abdominal shield ; / or fern, femoral ; g or gul, gular, 

 unpaired in 3 ; h or hum, humeral shield ; i or int.g, intergular ; im, infra- 

 marginals ; m, marginals; p or pect, pectoral; x (in 1) inguinal shield con- 

 stituting with the axillary xx, the last trace of inframarginals. (After 

 Gadow). 



In the trunk region the vertebrae are rigidly united to the narrow, 

 paddle-like ribs (Fig. 425). There are eight cervical, ten thoracic, two 

 sacral, and a variable number of caudal vertebrae, which are procoelous 

 in form (Fig. 404). A peculiar thing in the turtle is that both pectoral 

 and pelvic girdles are inside instead of outside the ribs. They actually 

 arise from primordia internal to the ribs so it is not a case of migration. 

 No one has yet been able to give a satisfactory explanation of this. 



The pectoral girdle (Figs. 416, 417) is made up of a triradiate group 

 of flattened bones : the scapula, the procoracoid, and the coracoid, the 

 last being the largest. These three bones unite to form a socket which 

 receives the head of the humerus. The pelvic arch is more compact. It 

 consists of pubis, ischium, and ilium, which unite to form the acetabulum 

 for the head of the femur. Membrane bones are never found in the pelvic 

 girdle of any animal. 



AVES 



The sternum is keeled (Figs. 416, 418), except in such birds as the 

 ostrich, and the ribs have uncinate processes (Fig. 418, B, u.p.) except in 

 Screamers (members of the family Palamedeidae). The trunk vertebrae 



