Glass 2. Pisces. 



363 



may be present but are less constant. In the Dipnoi some of these bones 

 occiu' biit cartilage persists to a large extent. In the cartilaginous Ganoids, 

 as already mentioned, only membrane bones are found, among them a parasphenoid, 

 frontals, parietals, and several smaller bones dorsaUy. 



Fig. 300. Skull of a Perch, A dorsal, B ventral. 1 frontal, 2 prefrontal, 3 

 ethmoid, 4 postfrontal, 5 basioceipital, 6 parasphenoid, 7 parietal, 8 supraoooipital, 9 epiotic, 

 10 exoooipital, 11, prootio, 12 squamosal, 13 opisthotic, 14 alisphenoid, 16 vomer. — After 

 Cuvier and Valenciennes. 



The dorsal membrane bones of the head are in many Fish very superficial in 

 position, covered only by a thin layer of connective tissue and epidermis (Stui-geon, 

 bony G-anoids, many Teleosteans) ; in others, the overlying connective tissue is 

 thicker. 



A large number of Tisceral arches* usually seven pairs, 

 occasionally more (some Sharks), are suspended from, or situated 

 near to, the skull. The members of the first pair are united ventrally, 

 whilst the others are attached to a series of unpaired, cartilaginous, 

 or bony pieces (basibranchials). The most anterior, the 

 mandibular arch, consists, in the Sharks, of an upper and a 

 lower cartilaginous piece ; the former, which is connected in front 

 with the corresponding one of the other side, is termed the p a 1 a t o - 

 quadrate, or less happily, the upper jaw; the latter, the 

 mandibular cartilage, or lower jaw. The two portions are 

 jointed together. The mandibular arch, the best developed, is loosely 

 connected with the skull in the Sharks, and forms the framework of 

 the mouth. The second, the hyoid, is similarly divided into two 

 parts ; the upper is fastened to the skull above, whilst its lower end is 

 attached by connective tissue to the mandibular. The other 



* The Cyolostomi are excluded from this <ieeeription of the visceral arches, 

 arrangements in this group are much modified and difficult to understand. 



The 



