INTRODUCTORY. JAWS AND VISCERAL SKELETON. 21 



The basicranial axis in front of the presphenoid is ossified,, 

 as the mesethmoid, dorsal to which there sometimes, as in 

 Teleosteans, occur a median ethmoid and a pair of lateral 

 ethmoids 1 . Two pairs of membrane bones very commonly 

 occur in this region, viz. the nasals which lie dorsal to the 

 mesethmoid, and the vomers (sometimes there is only one) which 

 lie ventral to it. 



The part of the skull lying immediately in front of the 

 cranial cavity and in relation to the nasal capsules constitutes 

 the ethmoidal region. 



There remain certain other membrane bones which are 

 often found connected with the cranium. Of these, one of 

 the largest is the parasphenoid which, in Ichthyopsids, is 

 found underlying the basicranial axis. Prefrontals often, as 

 in most reptiles, occur lying partly at the sides and partly in 

 front of the frontal, and postfrontala similarly occur behind the 

 orbit lying partly behind the f rentals and partly at their sides. 

 Lastly a squamosal bone is, as in Mammals, very commonly 

 developed, and lies external and partly dorsal to the auditory 

 capsules. 



THE JAWS AND VISCERAL SKELETON. 



In the most primitive fish these consist of a series of 

 cartilaginous rings or arches placed one behind another and 

 encircling the anterior end of the alimentary canal. Originally 

 they are mainly concerned with branchial respiration. 



The first or maxillo-mandibular arch forms the upper 

 jaw and the lower jaw or mandible. 



The second or hyoid arch bears gills and often assists 

 in attaching the jaws to the cranium. The remaining arches 

 may bear gills, though the last is commonly without them. 



The above condition is only found in fishes, in higher 

 animals the visceral skeleton is greatly reduced and modified. 



The first or maxillo-mandibular arch is divisible into a 



1 Sometimes also called ectethmoids or parethmoids. 



