I/O MORPHOLOGY OF THE ORGANS OP VERTEBRATES. 



and an anterior procoracoid, both extending inwards ; and fre- 

 quently the inner ends of these are united b}' a longitudinal 

 cartilaginous band, — the epicoracoid. To these may be added 

 a clavicle, developed from membrane, in front of the proco- 

 racoid, extending in- 

 wards from the scap- 

 ula, and usually con- 

 necting with the 

 sternum by means of 

 the episternum. 

 These parts undergo 

 various modifications, 

 and some or all of 

 them, with the excep- 

 tion of the scapula, 

 may, here and there, 

 more or less com- 

 p 1 e t e 1 y disappear. 

 Possibly the most 

 common is the re- 

 placement of the 

 procoracoid b\" the 

 clavicle. The details 

 of these modifications 

 be given in connection with the groups in which they 

 occur; but in the majority the two halves of the pectoral 

 girdle are more or less firmly united by means of the sternum. 

 The pelvic girdle presents many similarities to the anterior 

 arch. In the elasmobranchs there is the same transverse arch 

 as in the shoulder girdle ; and this supports the ventral fins, 

 there being in some cases a dorsal portion e.xtending beyond 

 the fossa (acetabulum) in which the fin articulates. There thus 

 arise a dorsal iliac portion and a ventral ischio-pubic portion in 

 each half of the arch, the ventral part being perforated b\- an 

 opening (obturator foramen) for the obturator nerve. In the 

 other fishes the pelvic girdle is much less developed, and in the 

 teleosts the fins are supported by these enlarged basal elements 

 (vide infra). In the amphibia and higher groups the iliac por- 



FiG. 182. 



C, clavicle ; 



Shoulder girdle of Ornithorhynchtis. 

 CO, coracoid; E, episternum; EC, 



epicoracoid; S, scapula; ST, sternum; R, ribs. 



will 



