CLASS AMPHIBIA 495 



parasphenoid. The lower jaw is composed of a pair of each, dentary 

 bones, which bear teeth; splenial bones, which bear the last few teeth; 

 and the angular bones, devoid of teeth and articulating with the 

 quadrate of the skull. Necturus usually has forty-six amphicoelous 

 vertebrae. They articulate with each other by anterior and posterior 

 zygapopJiy.ses as well as the ends of the centra. There is one cervical 

 vertebra, atlas, with which the skull articulates. Posterior to this 

 one are about eighteen thoracolumbar vertebrae each of which bears 

 a pair of short Y-shaped ribs. Each rib has a double head (bicipital) , 

 the dorsal head or tuherculum articulating with the transverse process 

 of the vertebra and the ventral head or capitulum articulating with 

 the side of the centrum. Following the thoracolumbar group is a 

 single sacral vertebra to which the ilium of each side is attached by 

 way of the sacral rib. The remainder of the series, posterior to this 

 point, consists of caudal vertebrae. 



The pectoral girdle is principally cartilage in structure. The ven- 

 tral portion is formed by a posterior coracoid cartilage in the muscles 

 of the body wall, and an anterior procoracoid. Projecting dorsally 

 and laterally is the third unit of each side, the scapula. The most 

 dorsal, free margin of this is frequently referred to as suprascapula. 

 The recess formed at the junction of scapula with the ventral parts 

 into which the arm articulates is called the glenoid fossa. The skele- 

 ton of the anterior appendage includes the proximal humerus (in the 

 brachium), the radius and idna in the forearm (antebrachium), six 

 carpals in the wrist, four metacarpals in the palm, and four digits 

 each composed of joints or phalanges. 



The pelvic girdle is likewise largely cartilage, but it is fused in the 

 midventral line. The anterior, ventral part is the puhic plate con- 

 sisting of cartilage, posterior to this is the pair of iscMa which are 

 partly ossified. Extending dorsally on each side is a slender ilium 

 which joins the sacral rib and this in turn the sacrum. In the lateral 

 position where the ilium meets the two ventral parts of the girdle is 

 a concave recess into which the head of the femur of the thigh articu- 

 lates. This recess is called the acetahidum. Distal to the thigh is 

 the shank with two bones, the tibia and the fihula lying parallel to 

 each other. There are six somewhat fused tarsals in the ankle. Distal 

 to this are the four elongated metatarsals and beyond each is the digit, 

 composed of phalanges.* 



•Illustrations of the skeleton of Necturus may be found in Stuart: Anatomy 

 of Nectwrus maculosus, Denoyer-Geppert Co., Chicago, 



