118 LECTURE V. 



the Plagiostome Cartilaginous Fishes it is not directly attached to its 

 proper vertebra, the occiput, but is removed further back, where we 

 shall usually find it displaced in the higher Myelencephala, in order to 

 allow of greater freedom to the movements of the head. 



The superior piece of the arch {supra- scapula, fig. 30, 50) is 

 rvmJ5A^v>j^ -Ijjfjjpgr^tg \y^ ^]^g Qq(]^ qj, consists of two short columnar bones, at- 

 tached anteriorly, the one to the par-occipital, the other and shorter 

 piece to the petrosal, and coalescing posteriorly at an acute angle, to 

 form a slightly expanded disk, from which the second piece of the 

 arch is suspended vertically. 



This second piece, called ^'•scapula" (ib.5\) is a slender, straight, 

 ^^ styliform bone terminating in a point below, and morticed into a 



groove on the upper and outer side of the lower and principal bone 

 of the scapular arch. The supra-scapula and scapula together repre- 

 sent the rib or pleurapophysis of the occipital vertebra ; they are 

 always confluent in the Siluroids. 



The lower bone, or hi^mapophysis {coracoid, ib. 52), which completes 

 the arch below, is commonly termed the ' clavicle' (as by Spix, Geoffroy, 

 Meckel and Agassiz) ; but I am induced to regard it as homologous with 

 that bone, the coracoid, which progressively acquires a more constant 

 and larger development in descending from Mammals down to Fishes, 

 and which is manifestly a more essential part of the arch than the 

 clavicle, since it conti'ibutes more or less of the surface of attachment 

 for the radiated appendage, which the clavicle never does. By Cuvier 

 the liEemapophysial portion of the occipital inverted arch in fishes 

 is termed the ' humerus ;' but it is unquestionably a part of the arch, 

 and the most important part in the present class, in no member of 

 which does it present the slightest approach to the character of a 

 diverging appendage, such as the humerus essentially is, whenever it 

 has an independent existence. By some Ichthyotomists the bone in 

 question has received the special name of ' coenosteon.' 



Whether viewed insulated, i. e. merely iclithyotomically, or by 

 the light of the modifications of the sustaining arch of the pectoral 

 member in higher Vertebrata, the essential nature of the bone in 

 question was little likely to be understood : its general homology can 

 only be appreciated by studying its relations to the general vertebrate 

 skeleton in the lowest class, where vegetative repetition most prevails, 

 and the fundamental type is least departed from. The relation to 

 the primary constituent segment of the skeleton being thus ascer- 

 tained, the special homology of the bone is to be determined by 

 tracing the modifications of tlie scapular arch in the ascending di- 

 rection. The serial homologies of the hasmapophyses of the scapular 

 arch are obviously with the cerato-hyoids, the mandible, the cartilages 



