SKULL OF VERTEBEATA. 455 



§345. 



The skeleton of the operculum is one of the most important of 

 the skeletal parts which are connected with, although they did not 

 primitively belong to, the mandibular apparatus. In the Selachii 

 there are cartilaginous pieces, which are sometimes bifurcated, in the 

 place of this bony skeleton; these are set on both segments of the hyoid 

 arch, where they form branchial rays. This cartSage is covered in 

 by a common membrane, as is also the bony apparatus; the mem- 

 brane is adapted to this latter, and converts it into a defensive 

 arrangement, which extends over the postjacent branchial clefts. 



In the Sturiones the first to appear is the largest of these bones 

 — the operculum ; and to it, in the rest of the Granoids, and in the 

 Teleostei, the other bones are attached (Fig. 245). The preoper- 

 culum {Pr Op) is developed in the cartilage which connects 

 together the hyomandibular and symplectic. It is often more closely 

 united to the constituent parts of the suspensorium. Behind 

 the preoperculum is the suboperculum {Sop), which is present, 

 in addition to the operculum, in Ceratodus ; and then below it there 

 is the interoperculum (Jop), which is connected with the lower 

 jaw by a ligament (Jig). 



Accessory bones are formed from other pieces, which are 

 developed from parts of the dermal skeleton ; the most important 

 of these are the infi-aorbital bones (cf. Fig. 245, i i i i). They form 

 a curved series around the lower edge of the orbit ; the hindermost 

 piece is attached to the postfrontal, and the foremost to the lateral 

 ethmoid. Some of these bones acquire a considerable size in the 



Cataphracta (Trigla) . 



The nasals also belong to this series of bones, in consequence of 



their inconstant presence ; there are also many other pieces that are 



connected with the so-called mucous canal system, and which are 



modifications of scales. 



Veoijk, a. J., Ueber die Verknocherung u. die Kuochen des Schiidels del' 

 Teleostei. Niederland. .Aa-chiv f. Zoologie. I.— Parker, W. K. Develop^ 

 ment of the Skull in the Salmon. Philos. Transact. 1873. 



§ 346. 



In the skull of the Amphibia the primordial cranium is some* 

 times greatly developed. It very frequently, however, loses its roof, 

 and also its floor, owing to the formation of spaces in the cartilage. 



The palato-quadrate is directly connected with the primordial 

 cranium ; it is attached posteriorly to the auditory capsule of the 

 skull, while anteriorly it forms an arch around the orbits, and either 

 projects freely forwards (as in the Urodela), or is connected with 

 the cranium in the ethmoidal region. Behind, and at the sides, it 

 carries the glenoid cavity. It presents therefore those relations 

 which were seen in the Chimferte, and in the Dipnoi ; and there are, 



