jo4 



THK FKOG. PHYLUM CHORD ATA 



bone. Above, there abuts on its outer side a T-shaped mem- 

 brane bone known as the squamosal which touches it by one 

 Innb of the cross-piece of the T, the main limb being directed 

 outwards and downwards. At one spot on the outer side of the 

 capsule a separate piece of cartilage tits into a membrane-covered 

 gap known as the fenestra ovalis, and from it a slender rod of 

 bone and cartilage, homologous with the hyomandibula of fishes, 

 but here called the columella auris, runs to the drum of the ear, 

 so that when the latter is thrown into vibrations by sound waves 



Fig. 269. 



-Skull of frog, side view. — From Young, The Life of Vertebrates, 1950. 

 Clarendon Press, Oxford.— After :Marshall. 



ac., .'Vnterior horn of hyoid ; art., articular ; as., angulosplenial ; b.h., body of hyoid ; col., columella auris ; 

 d., dentary ; ex., e.xoccipital ; fp., frontoparietal ; m., maxilla ; mm., mentomeckelian ; n., nostril ; 

 na., nasal'; par., parasphenoid ; ^.c, posterior horn of hyoid ; pm., premaxilla ; pro., pro-otic ; pt., 

 pter>-goid ; q., quadrate; qj., quadratojugal ; se., sphenethmoid ; sq., squamosal; //, IX, and X, 

 foramina for cranial nerves. 



its movements are transferred by the columella through the 

 membrane to the inner ear w^hich lies beneath it. 



The nasal capsules are a pair of irregular, mainly cartilaginous, 

 enclosures continuous with the front end of the cranium. Only 

 their hinder part is ossified, and this forms that part of the 

 sphenethmoid which lies in front of the transverse partition of 

 the latter. The wall between the two capsules is know^n as the 

 mesethmoid. Through these capsules run the passages from the 

 nostrils to the mouth, and each of them has therefore an opening 

 above and below. Each bears two membrane bones, one on its 

 upper side and one beneath. The upper bone is known as the nasal, 

 and is shaped hke the outhne of a pear, with the stalk directed 

 outwards. The lower, usually called vomer, is also called prevomer, 

 because of doubt as to its homology w'ith the vomer of mammals. 

 It is of irregular shape and carries a patch of teeth which project 

 through the skin of the roof of the mouth. 



