VERTEBRATES. 



19 



Fig. 22. — Skull of snake (Tropidonotus) ; an, 

 angular ; «o, antorbital ; ar, articular ; d, 

 dentary ; eo, exoccipital ; /, frontal ; p, 

 parietal ; pi, palatine ; pm, premaxlUary; 

 pr, prootic ; pt, pterygoid; q, quadrate; sa, 

 surangular ; sh, styloliyoid ; so, supraoccip- 

 ital ; sq, squamosal. 



It has been mentioned above that the upper part of the second or Iiyoid arch 

 possesses the function in many fishes of supporting partly or entirely the man- 

 dible. Such skulls have been called liyostylic, in contradistinction to those auto- 

 stylic skulls, where the quadrate alone performs 

 this duty. The hyoid suspensorium, when present 

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

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

 mandibular and lower symplectic, which ossify sep- 

 arately. Connected 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- 

 dlbular. Generally four bones are present in it, 

 the preopercular, opercular, subopercular, and the 

 interopercular ; the first of these is developed 

 round a neuroinastic canal, while the others ap- 

 pear to be similar to the branchiostegal rays 

 which support the membranous fold developed 

 from the lower part of the hyoid ai-ch, and assist- 

 ing in the protection of the gills (Fig. 21). 



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 

 Bubsei"vieut 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 in the amphibians, reptiles, and birds. Some doubt is 

 entertained by morphologists 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 have 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 ear ; 

 the squamosa], 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 

 asserted that the malar bone of mammals is likewise of complex 

 nature, representing 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 

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



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

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

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



Fig. 23.— Auditory region 

 of human fcetus; i, incus; 

 I, lower jaw; m, processus 

 gracilis of the malleus 

 joining the Meckeliaii 

 cartilage of the lower 

 jaw ; ml, malleus ; s, 

 stapes, within the tym- 

 panic ring; z, zygomatic 

 arch of temporal bone, 

 bearing the glenoid fossa. 



