XIII 



PHYLUM CHOEDATA 



185 



n.sp 



Tt.a, 



c.r 



rich 

 nc7i.sh 



J 



very characteristic form, largely owing to the compression of the 

 region between and in front of the large orbits, which are separated 

 from the cranial cavity only by membrane in Callorhynchus (Fig. 

 861, or.) ; in Chimsera they lie above the level of the cranial cavity 

 and are separated from one 'another by a median vertical partition 

 of fibrous tissue (Fig. 860, i. o. s). At first sight the palatoquadrate, 

 or primary upper jaw, appears to be absent, but a little consideration 

 shows it to be represented by a triangular plate (pal. qu.) which 

 extends downwards and outwards from each side of the cranium 

 and presents at its apex a 

 facet for the articulation of 

 the mandible. The palato- 

 quadrate is therefore fused 

 with the cranium and fur- 

 nishes the sole support for 

 the lower jaw ; in a word 

 the skull is autostyUc. The 

 pituitary fossa (Fig. 861,5. t.) 

 is very deep and inclined 

 backwards ; on the ventral 

 surface of the basis cranii is 

 a pit (pt.) for the extra-cranial 

 portion of the pituitary body. 

 The posterior portion of the 

 cranial cavity is very high ; 

 the anterior part containing 

 most of the fore-brain is 

 low and tunnel-like, and has 

 above it a cavity of almost 

 equal size (Nv. 5 o'.) for the 

 ophthalmic branches of the 

 fifth nerves. The greater 

 part of the membranous laby- 

 rinth is lodged in a series of 

 pits on the side-walls of the 

 cranium (a.s.c., p.s.c.), and is 

 separated from the brain by 

 membrane only. The occipital region articulates with the vertebral 

 column by a single saddle-shaped surface or condyle (oc. en.). 

 There is a great development of labial cartilages, particularly 

 noticeable being a large plate w T hich, in Callorhynchus, lies just 

 externally to the mandible, nearly equalling it in size and having 

 the appearance of a secondary or external jaw. In Callorhynchus the 

 snout is supported by three cartilaginous rods growing forward from 

 the cranium, of which one (r.) is median and dorsal and represents 

 the rostrum ; these, as well as the great lower labial, are represented 

 by comparatively small structures in Chimaera (Fig. 860, ?&.-/). 



B 



7Z.fl 



FIG. S.")0. Chimaeramonstrosa. .4, transverse 

 section of the vertebral column ; B, lateral view 

 of the same. c. r. calcified ring ; h. r. hrem; 1 

 ridge ; int. intercalary piece ; n. a. neural arch ; 

 nch. position of notochordal tissue ; nch. sh. 

 sheath of notochord ; n. sp. neural spine. (After 

 Hasse.) 



