HIGH SCHOOL ZOOLOGY. 



27 



(Fi". 10) which in its timi finds a firm Imsis of resistance in the 

 skull and v(!rtcbral column by means of the supraclaviclc or 

 post-temporal described above. This bone is firmly wedged into 

 a socket in the clavicle proper which lies very close to the skin, 

 (especially immediately above the fin where a rough process 

 projecting backwards from it may be felt), and is regarded as a 

 skin-bone, formed on the substructure of the primary arch 

 which it conceals and with which it is closely united. The 





Fig. 10.— Peotoral Girdle of Catfish from behind. 

 Co, coracoidal, s, soaimlar portion of jnimary shonlder-<,'irdle ; el, clavicular, 

 so, supra-ola\ ifular portions of secondary- shoulder-girdle ; 1), liasal elements, r, rays of 

 the fill-skeleton. 



latter is at first cartilaginous, the former never so. Both the 



clavicle and the primary shoulder-girdle are di\ ided into upper 



and lower parts by the fin, the lower parts uniting in the 



veintral middle line and thus completing the arch. The upper 



part of the primary arch is known as the scapula, the lower 



as the coracoid, and the region wliere the fin unites with it the 



glenoid region. 



2 ■. Let us now examine the skeleton of the fin itself. It is 



made up of fin-rays of which the anterior is bony throughout, 



and toothed on its posterior margin ; it is a hard ray or 



