330 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



(Fig. 139 ; par). The upper half of the mandibular 

 arch becomes invested by membrane bones only, the 

 jug-al or quadratojugal, or both, which trend a 

 little inwards as they pass forwards (Fig. 139 ; q). 

 In front of these are two bones which, typically, carry 

 teeth, the maxillae and the premaxilla^. 



Internal to this row of membrane bones is another, 

 of which the most anterior, the vomer (v), is formed 

 from membrane that did not overlay cartilage, just 

 like the maxillae and the premaxillse, while the 

 others, the palatines and the pterygoids (pt), are 



formed from mem- 

 brane which gene- 

 rally invested carti- 

 laginous bars. Mec- 

 kel's cartilage is 

 likewise invested in 

 bones of membra- 

 nous origin, the most 

 important and con- 

 stant of which is 

 the tooth - bearing 

 dentary. At the 

 anterior end of the 

 skull, above the 

 olfactory capsule, 

 there appear the 

 paired nasals, with 

 which a lachrymal 

 is related in the 

 higher forms. 



In Fishes a series of membrane bones may become 

 developed in connection with the branchial skeleton, 

 and form the support for the opercular flap of the 

 gills ; such are the operculum, siiboperculiiin, 

 and interoperciiliim. The most anterior of the 

 opercular bones is possibly the homologue of the 



Fig. 139. Skull of Frog, from below; 

 the Lower Jaw has been removed. 



e,o, Exoccipital ; po, prootic ; par, parasphe- 

 noid ; e, sphenethnioid ; r, vomer; pm, 

 premaxilla- mx, maxilla; q, quadrato 

 jueral ; pt, pterygoid ; sus, suspenBorium ; 

 palatine; 1, optic foramen; 2, foramen 

 of fifth nerve ; 3, foramen for ninth and 

 tenth nerves. (After Parker.) 



