THE SKULL OF OSSEOUS FISHES. 119 



of the ribs, or sternal ribs, &c. The modification of the coracoids, 

 most characteristic of fishes, is the sympliysial union of their lower 

 extremities, like the haemapophyses of the maxillary and mandibular 

 arches, either by ligament, or dentated suture ; or, as in Plagiostomes, 

 by cartilaginous confluence. But tliis mode of closing the inverted 

 arch seems inevitable in a class in which a true sternum ' hcemal 

 spine' is absent; and we shall find the same symphysis of the cora- 

 coids in those fisli-like Reptiles, the EnaUosauria, which are now 

 extinct. 



In the Cod-tribe the pointed upper extremity of the coracoid 

 {figs. 19. and 30. 52), projects behind the scapula and almost touches 

 the supra-scapular bone ; below this part a broad angular plate of the 

 coracoid projects backwards and gives attachment to the radiated 

 appendage of the arch : the rest of the coracoid bends in^vards and 

 forwards, gradually decreasing to a point, which is connected by 

 ligament to its fellow, and to the uro-hyal bone. The inner side of 

 the coracoid is excavated, and its anterior margin folded inwards and 

 backwards ; it is continued above into the posterior angular process, 

 but in the rest of the coracoid it is simply bent upon the inner con- 

 cavity of the bone, which lodges the origin to the great lateral muscle 

 of the trunk. 



In most fishes the lower end of the arch is completed, as in the 

 Cod, by the ligamentous symphysis of the coracoids ; but in the 

 Siluri and Platycephali the coracoids expand below, and are firmly 

 joined togetlier by a dentated suture. In all fishes they support and 

 defend the heart, and form the frame, or sill, against which the oper- 

 cular and branchiostegal doors shut in closing the great branchial 

 cavity ; they also give attachment to the aponeurotic diaphragm 

 dividing the pericardial from the abdominal cavity. 



Like the tympano-mandibular and hyoidean arches the scajmlar 

 arch supports, in most fishes, a Diverging or radiated Appendage 

 on each side. 



This appendage consists in Lepidosiren of a single ray ; but in 

 Osseous Fislies it is composed usually, first, of two rarely of three 

 bones immediately articulated with the coracoid ; next, of a series of 

 from two to six smaller bones ; which, lastly, support a series of 

 spines or jointed rays. These rays in the scapular appendage, or ' pec- 

 toral fin,' are a repetition of the branchiostegal rays in the hyoidean 

 appendage, and of the opei'cular rays in the tympanic appendage. 

 Of the special homology of the pectoral fin-rays with the digits of 

 the jiectoral extremity in higlier animals, there has been no question. 

 The vegetative repetition of digits and joints, and the vegetative 

 sameness of form in those multiplied peripheral parts of the fins of 



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