SKULL OF VERTEBRATA. 



453 



wliicli passes obliquely backwards from the orbit, this bone forms a 

 pillar between the canals of either side. Not unfrequently it seems 

 to disappear altogether. On the ventral side of, and along the pri- 

 mordial cranium there extends the large parasphenoid (Figs. 242, Ps ; 

 243, 5), which has been already seen in the Sturiones. 



It is seldom that the whole of the primordial cranium is retained 

 on the upper surface ; as a rule, there are spaces in it, which are 

 overlaid by covering bones. Nearest to the posterior primary region 

 there are the two parietals (Fig. 244, 7) ; these are sometimes sepa- 

 rated from one another by an anterior process of the supra-occipital 

 (3). In front of them there are the frontals, which are often re- 

 placed by a united frontal (11). At the sides of this are the two post- 

 frontals (12), which extend as far as the squa- 

 mosal, and take part in the articulation of the 

 hyomandibular. 



In the ethmoidal region there is a median 

 piece, ethmo'idale medium (1(3), to the sides of 

 which the lateral ethmoids are attached (14) 

 (frontalia anteriora. Cuvier). The latter 

 form the base of the nasal capsules. The 

 median portion of the ethmoids is often per- 

 manently cartilaginous. The vomer forms the 

 covering piece of the base of the ethmoidal 

 region ; posteriorly this is united to the para- 

 sphenoid ; it is paired in Lepidosteus. 



§ 344. 



Fig. 244. Skull of a 

 Gad us from above. 

 3 Supra-occipital. 4Epi- 

 otic. 6 Squamosal. 



7 Parietal. 11 Frontal. 

 12 Postfrontal. 14 La- 

 tcral ethmoid. 16 Me- 

 dian ethmoid. 



The mandibular apparatus of the Selachii 

 is not completely retained in the Gano'idei and 

 Teleostei, owing to the presence of bony struc- 

 tures in its place. It is complicated by the 

 union of the hyomandibular with the bones 

 formed from the palato- quadrate cartilage. 

 The primitive relations, as they are seen in the 



embryonic condition, show that it has been derived from the arrange- 

 ments which obtain in the Selachii. While the anterior ends of the 

 right and left palato- quadrate bones are connected by ligament in the 

 Selachii and Sturiones, in the rest of the Gano'idei and in the Tele- 

 ostei they are attached to the sides of the primordial cranium, and 

 are separated from one another by the ethmoidal region, the basal 

 surface of which enters into the formation of the boundaries of the 

 buccal cavity. 



The hyomandibular (Fig. 245, Hm) is almost always a large bone, 

 which articulates with the squamosal and postfrontal. The sym- 

 plectic (Fig. 241, s) is segmented off from it; in the Selachii this is 

 represented by a process, but in the Sturiones it is an inde- 

 pendent piece ; the lower portion of the hyoid arch is inserted at 

 the point where the symplectic is connected with the hyomandibular. 



