124 



VERTEBRATE SKELETON 



between capsule and the occipital vertebra, the arch of which soon meets the 

 capsule above the nerce, completing the jugular foramen. The capsule is 

 divided internally for the semicircular canals. The relations of the stapes to 

 the fenestra has been alluded to (p. 119). 



The large fenestra hypophyseos may be increased later by resorption of the 

 cartilage separating it from the openings in the basal plate, so that a common 

 basicranial fenestra results. The two trabeculse do not meet in front at first, 

 but later their union forms the ethmoid plate, beyond which they continue as 

 the cornua trabecularum beneath the otic capsules. The ethmo-nasal region 

 differs in the various genera, details of which must be sought elsewhere. The 



Fig. 131. — Chondrocranium of Amphiuma. c, cornu trabeculae; bf, basal fenestra; 

 d, endolymph duct foramen; e, ethmoid plate; ep, epipterygoid; /z;; fenestra vestibuli; 

 rn, Meckel's cartilage; w, notochord; oc, otic capsule; p, parachordal; po, postotic pillar; 

 q, quadrate; s, stapes; si, sphenolateral; /, trabecula; 2-8, nerve exits. 



capsules of the two sides are separated in front by an internasal space, while 

 farther back is a septum, differing in several respects from that of other Verte- 

 brates. Internally the capsules largely lack the projections which, in higher 

 groups, are indicative of complication of the olfactory membrane. 



The visceral arches (six at most) appear in order, from in front backwards. 

 The mandibular arch divides at chondrification into pterygoquadrate and 

 Meckelian. The former part, at first almost entirely quadrate, soon joins the 

 trabecular wall by the epipterygoid process and later basal and otic processes 

 connect it with basal plate and optic capsule. The skull thus becomes moni- 

 mostylic, the monimostyly, like that of Dipnoi, being effected by cartilage union, 

 and differing from that of reptiles where fixation of the quadrate is largely by 



