THE ENDOSKELETON : GIRDLES, THE STERNUM, AND APPENDAGES 91 



Each half of the girdle consists of two ventral parts and one dorsal part. The 

 anterior ventral part is not an independent bone but a projection from the 

 scapula, and is hence named the proscapular process (erroneously referred to in 

 some texts as the procoracoid bone). The posterior ventral bone, the larger of 

 the two, is the coracoid (which as already explained is probably in reality the 

 procoracoid). The dorsal bone, an elongated bone reaching to the carapace, is 

 the scapula. All of these are cartilage bones. In addition there are certain 

 bones of the pectoral girdle which are included in the plastron and are there- 

 fore membrane bones. These are the paired clavicles (epiplastra) and the 

 median inter clavicle (entoplastron). Refer to your drawings of the turtle 

 plastron. 



The pectoral girdle of the alligator consists of a stout dorsal bone, the scapula; 

 a stout ventral bone, the coracoid ; and a long, slender dagger-shaped bone in the 

 median ventral line, the interclavicle. Clavicles are lacking. A sternum is 

 present. It is composed of a plate of cartilage between the ventral ends of 

 the coracoids and just above the interclavicle; it is drawn out posteriorly into 

 long curved cartilages, the xiphisternal horns. Observe that the ribs are attached 

 to the sternum by means of their sternal ribs, a condition first met with in 

 reptiles. 



The bones of the fore limbs are the same as those of Necturus in general. The 

 bones of the carpus or wrist of the turtle are remarkably primitive in number 

 and position, and deserve further attention. If not sufficiently clear on the speci- 

 men, expose them by cautiously picking away the dried ligaments covering them. 

 At the base of the ulna are two bones, an outer ulnare, and an inner intermedium. 

 The center of the carpus is occupied by a long bone which is the fused radiate (end 

 at the base of the radius) and the centrale (larger end). Distal to this is a row 

 of five carpales, one for each metacarpal. This arrangement is therefore very 

 much like that of the ideal carpus. Figures of the carpus of turtles are given 

 in Wd, pages 163, 164. Draw, showing the bones of the carpus. 



7. The pectoral girdle, sternum, and fore limb of birds. The pectoral 

 girdle of birds is a very complete and generalized girdle. It is composed of the 

 scapula, a long swordlike bone lying above the ribs; the coracoid, a stout bone 

 reaching the sternum; and the wishbone, or furcula, in front of the coracoids and 

 attached at the lower end by ligaments only. The furcula is membrane bone 

 and really consists of two clavicles, the two forks of the wishbone, united to a 

 rounded piece, the interclavicle. Coracoid and scapula take part in the forma- 

 tion of the glenoid fossa. The sternum of birds is highly specialized. It is an 

 elongated bone bearing a strong ventral projection, the keel or carina. The carina 

 serves for the attachment of the powerful wing muscles. As in reptiles, the ribs 

 are joined to the sternum by their costal cartilages. The front end of the sternum 

 has short costal processes, each side two long xiphisternal processes. Draw from 

 the side, showing girdle and sternum. 



