562 FISHES. 



resistance to the strong weapon witli whicli it is frequently 

 armed. The post-temporal, as we have said above, is often 

 united by suture to the cranium, and it obtains support below 

 by one or two processes that are fixed on the basioccipitals 

 and on the diapophysis of the first vertebra. 



In most osseous fishes the clavicle completes the lower 

 key of the scapular arch ia joining its fellow by suture or 

 synchondrosis without the intervention of the coracoid ; but 

 in the Siluroids the coracoid descends to take part in this 

 joint, and sometimes even to occupy the half of the suture, 

 which is not unfrequently constructed of very deep inter- 

 locking serratures. The solidity of this base of the pectoral 

 spine is further augmented by the intimate union of the 

 coracoid and scapula, which often extends to junction by 

 suture, or even to coalescence ; and these bones, moreover, 

 give off two bony arches — the first a slender one, arising from 

 the salient edge of the coracoid near the pectoral fin, and 

 going to the interior face of the scapular that is applied to 

 the interior surface of the ascending branch of the clavicle ; 

 the second and broader supplementary arch is often perforated 

 by a large hole ; it also emanates from the same salient edge 

 of the radius, but proceeds in opposite direction to the inferior 

 edge of the clavicle, a little before the insertion of the pectoral 

 spine. The two arches give attachments to the muscles that 

 move this spine ; in the Synodontes and many Bagri the 

 upper arch remains in a cartilaginous or ligamentous condi- 

 tion, while in Malapterurus it is the lower arch that does not 

 ossify, but both are fully formed in the Siluri and many other 

 Siluroids more closely allied to that typical genus. The 

 post-clavicle is also wanting in the Siluroids. The pterygoid 

 and entopterygoid are reduced to a single bone, the symplectic 

 is wholly wanting, and the palatine is merely a slender cybn- 

 drical bone. The sub-operculum is likewise constantly absent 

 in all the Siluroids. 



