SKULL — AMPHIBIA 



131 



cranium, the complete pterygoid and zygomatic arches, the latter 

 bounding a temporal fossa, and in the presence of a quadratojugal 

 bone. 



The chondrocranium is more fully developed than in other Amphibia, and 

 at metamorphosis it undergoes extensive modifications, probably correlated 

 with changes in food, the tadpole (larva) scraping minute vegetation from 

 submerged objects with the horny jaws of the small mouth, the carnivorous 

 adult bolting large objects. As a result of the food, 

 the prechordal parts are first to appear (fig. 140). 

 There is no cranial flexure, the trabeculae lying in the 

 plane of the notochord, and extending forwards to meet 

 a suprarostral plate on either side (possibly the homo- 

 logue of the upper labial of Elasmobranchs), which 

 supports the horny jaws and appears before parts 

 farther back. Behind this plate the trabeculae meet 

 in an ethmoid plate. When parachordals and otic 

 capsules are outlined, the trabeculae join the basal plate 

 arising from the former, enclosing a large fenestra 

 hypophyseos. 



The jugular foramen is completed by the junction 

 of the pillar-Hke occipital vertebra with the otic cap- 

 sule and about the same time a sphenolateral bar 

 (taenia marginalis) arises in the interorbital region, 

 uniting with both trabecula and otic capsule, thus completing the foramen 

 lacerum for the V, VI and VII nerves. Later, the floor of the cranial cavity is 

 completed by growth from the surrounding parts, while additions are made to 

 the side walls from the anterior end of the sphenolateral and also from where 



Fig. 140. — Early 

 chondrocranium of 

 Rana (Stohr, '81). ct, 

 trabecular cornu. e, 

 ethmoid plate; ep, epip- 

 terygoid; n, notochord; 

 p, parachordal; pq, 

 pterygoquadrate. 



Fig. 141. — Chondrocrania of Rana. A, larva with tail disappearing (Parker, '70); 

 B, just after metamorphosis (Gaupp, '93). al, alary cartilage; c, trabecular cornu; 

 e, epipterygoid; et, ethmoid region;/?/, fenestra vestibuli; /, lower labial; m, Meckelian; 

 oc, otic capsule; pq, pterygoquadrate; pt, pterygoid; q, quadrate; s, stapes; t, trabecula; 

 ttn, median taenia; tn, ts, tectum of nasal and synotic regions; tp, terminal plate of nasal 

 capsule. 



the pterygoquadrate joins the cranium in front. The roof is never complete, 

 but in older larvae a transverse bar is connected with the synotic tectum by a 

 median cartilage, these parts separating a pair of fenestra; from a larger anterior 

 fontanelle. The compHcated nasal capsules need not be described here. 



