XIII 



PHYLUM CHORDATA 



253 



a bar of cartilage, having a membrane bone, the clavicle (Cl), 

 closely applied to it. The supra-scapula overlaps the anterior 

 vertebrae; the coracoid and procoracoid are connected ventrally 

 by a cartilage, the epicoracoid (Co') which is in close contact with 

 its fellow of the opposite side in the middle ventral line, so that 

 the entire shoulder-girdle, like that of the Dog-fish, forms a 

 single inverted arch. 



Passing forwards from the anterior ends of the united epi- 

 coracoids is a rod of bone, the cpisternum (Jfy.), tipped by a 

 rounded plate of cartilage, the omosternum ; and passing backwards 

 from their posterior ends is a similar but larger bony rod, the 



Kn, 



FIG. 873. Rana esculenta. The shoulder girdle from the ventral aspect. Co. coracoid; 

 Co.' epicoracoid ; CL clavicle ; G. glenoid cavity ; Ep. episternum ; Fe. fenestra between' 

 procoracoid and coracoid ; KC. cartilage separating scapula and clavicle ; Kn. xiphi-sternum ; 

 in, junction of epicoracoids ; Om. omosternum ; s. scapula ; st. sternum. (From Wieder- 

 sheim's Comparative Anatomy.) 



sternum (st\ also tipped by a cartilaginous plate, to which the name 

 xiphisternum (Kn) is applied. These two structures are the first 

 indication of a sternum we have yet met with, with the possible 

 exception of the median ventral element of the shoulder-girdle of 

 Notidanus (p. 162). The omosternum is developed as paired 

 forward extensions of the epicoracoids which undergo fusion : the 

 sternum and xiphi-sternum arise as paired rods lying posterior 

 to the epicoracoids, and subsequently uniting with one another. 



The fore-limbs deviate from the typical structure (p. 76) chiefly 

 in the fusion of the radius and ulna into a single radio-ulna 

 (Fig. 870, RA. UL), and in the presence of only four complete 

 digits with a vestigial one on the radial side. In all probability 



