VEHTEBUArKS. 



19 



-Skull of snake [Tropidonotns); an. 



delitary ; eo, i 

 parietal ; ;)/, \ 

 pr, prootic ; y/, 

 suranguhir ; .s7/ 

 ital ; sq, squaiii 



. It has been mentioned above that the upi)er jiart of the sccoiiil or hyoid arch 

 possesses the function in many fishes of su]>iiorting partly or entirely the man- 

 dible. Such skulls have been called hyostylic, in rontradistinction to those auto- 

 stylic skulls, where the quadrate alone perforins 

 this duty. The hyoid suspensorium, when present 

 as such, maybe a single stout bar, as in the sharks, 

 or may be divided into two jiieces, an upper hyo- 

 mandibular and lower symplectic, which ossify sep- 

 arately. Connei^ted with the interval between 

 these is the lower part of the hyoid arch, by a 

 bony or cartilaginous bar, the interhyal (Fig. 17). 

 In those fishes where a gill-cover is developed 

 (Ganoids and Teleosts) the skeleton of the gill- 

 cover is very intimately related to the hyoman- 

 dibular. Generally four bones are present in it, 

 the preopercular, opercular, subopercular, and the 

 interopercular; the first of these is developed 

 round a neuromastic canal, while the others ap- 

 pear to be similar to the branchiostegal rays 

 which support the membranous fold developed 

 from the lower pait of the hyoid arch, and assist- 

 ing in the protection of the gills (Fig. i!l). 



The gill-cover is absent in the Amphibia and higher classes, and the mandible is 

 suspended by the quadrate, but we have still to look for the homologue of the hyoid 

 suspensorium, which is now to be found as a chain of bones more or less numerous, now 

 subservient to hearing, and effecting communication between the membrane of the 

 drum of the ear and the internal ear. This chain is the columella auris, which exhib- 

 its much difference in its form hi the amphibians, reptiles, and birds. Some doubt is 

 entertained by niorphologists as to the homology of the chain of bones which perform 

 the same function in mammals, and this doubt extends to the nature of the articula- 

 tion of the lower jaw in that group. Unlike the lower classes, 

 where the mandible is articulated with an independent quad- ' 



rate, the mammals Lave the socket for the lower jaw on a bone 

 of very complex nature, the ' temporal ' bone. This bone not 

 only contains all the bones of the auditory capsule, but also a 

 tympanic developed in connection with the drum of the car ; 

 the squamosal, which is separate in the lower forms, and a 

 'zygomatic' process of that part the root of which bears the 

 glenoid socket for the mandible (Fig. 23). It has recently been Fio. 23.- Auditory resion 



-,.,.,. fi of luiman foetus; i, inous; 



asserted that the malar bone ot mammals is likewise of complex 



nature, rejn-esenting the postfrontal, jugal, and quadrato-jugal 



of reptiles ; further that the zygomatic process of the squamosal 



with which it articulates is nothing else than the quadrate of 



the lower classes; if so, the articulation of the lower jaw is 



tiie same in mammals as in lower forms, and this view derives 



support, according to Professor Cope, from the condition of affairs in the thero- 



morphous reptiles. On the other hand it lias generally been believed that the 



quadrate is represented in mammals by the malleus, one of the chain of bones in the 



/. lower jaw; m, proct^ssus 

 gracilis of the malleus 

 joining the Meckelian 

 cartilage of the lower 

 jaw ; ml, malleus ; x, 

 stapes, within the tym- 

 panic ring; z, zygomatic 

 arch of temporal bone, 

 bearing the glenoiii fossa. 



