SKULL. 



2G9 



may even be inclined backwards. The lower jaw presents articular, 

 dentary, and splenial elements. Teeth may be present on the premaxiUae, 

 maxillae, vomeropalatines, and parasphenoid, and on the lower jaw. 

 'Tlie visceral arches are generally reduced to the hyoid and two branchials 

 in the adult, but sometimes four (Siren, Amphiuma, etc.) or three (Nec- 

 turus, Proteus) pairs of branchials persist in the adult (Fig. 145 bis. A). 

 There is no tympanic cavity in Urodeles. 



In the Anura (Fig. 146) the cartilage of the cranium is much more 

 developed, there being a complete cartilaginous floor and roof, which 

 latter however contains some fontanelles. The two orbitosphenoids are 

 replaced by a single "girdle" bone which extends into the hinder part of 

 the ethmoid region 

 and is called the 

 sphenethmoid (Et). 

 The parietal and 

 frontal (Fp) are 

 fused and the 

 parasphenoid is 

 dagger-shaped IPs). 

 The suspensorium 

 slopes backwards 

 and the palatopte- 

 rygoid arch persists 

 as a bar extending 

 from the ethmoid 

 to the suspensorum. 

 Quadrato-jugals (.7) 

 are present, reach- 

 ing from the max- 

 illa to the quadrate. 

 The lower jaw^ con- 

 tains a mento- 

 meckelian cartilage 

 bone at the sjTn- 

 physis (o s s i fi e d 

 mento meckelian 

 cartilage of the 

 larva). A tym- 

 panic cavity, com- 

 municating with the pharynx by a eustachian tube is present, and the 

 stapedial plate is connected by a cartilaginous, partly ossified, rod, the 

 columella auris, with the tjTnpanic membrane, which is supported by a 

 cartilaginous ring. Teeth are found on the premaxillae, maxillae, vomers, 

 and the lower jaw% though the latter is frequently edentuloiis The 

 visceral arches (Fig. 145 bis, B) of the adult are represented by a large 

 basilingual plate in the floor of the mouth which is connected with the 

 otic region of the skull by the curved hyoid arches. All the four 

 branchial arches and part of the copula of the larva completely disappear, 

 the processes (including the bony thyTohyals) on the basilingual plate of 

 the adult being new formations.* 



As has already been stated, short two-headed ribs are often 



Fig. 145 bis. — Visceral arches of, A Trilmi criatatui ; B Rana tern- 

 poraria ; C tadpole of Rana ; D Siredon piiciformis (from Reynolds). 

 The bone is shaded and the cartilage left white. 1, basilingual 

 plate ; 2, hyoid arch ; 3, first, 4, second, 5, third, 6, fourth bran- 

 chial arch ; 7, thyrohyal ; 8, copula. 



* Ridewood P. Z. S., 1897, p. 577. 



