SKULL DIPNOI 



115 



connected respectively with pharyngo- and epibranchials. Usually 

 all of the arches bear wart-like or longer projections ('gill-rakers' 

 or 'strainers') on their anterior and posterior sides, these serving 

 to strain small particles taken in with the water through the mouth. 



DIPNOI. — The development of the cranium (figs. 120, 121) is known only 

 in Ceratodus, where in the earliest stage described the parachordals e.xtend as far 

 as the tip of the notochord but do not reach the 

 otic capsules. The orbital walls are formed by 

 trabeculae and sphenolateral cartilages, neither 

 extending forwards beyond the optic nerve. The 

 otic capsules are still procartilage and the rod-like 

 pterygoquadrate touches the cranium with its 

 anterior end at the junction of basal plate and 

 trabecula. In a much later stage the chondro- 

 cranium is a connected whole, except that the three 

 occipital vertebras are continuous with only the 

 epichordal cartilage. Sphenolaterals have now 

 united with the otic capsules above the fifth and 

 seventh nerves, the trabeculas below with the basal 

 plate. There is no trace as yet of a cranial roof 

 and the ninth nerve passes through the otic cap- 

 sule. The interorbital wall is continuous, the tra- 

 beculae extending to the tip of the snout. Near the 

 orbit is a process (posterior labial of Huxley, Sewertzoff's ethmoidal process) 

 directed outwardly and downwards, which persists in the adult. The trabeculae 

 pass medial to the olfactory organs and fuse in front. Two processes given off 



Fig. 120. — Early chon- 

 drocranium of Ceratodus 

 (Sewertzoff, '02). e, eye; 

 oc, procartilage of otic cap- 

 sule; ov, otic vesicle; p, 

 parachordal; q, quadrate; 5, 

 spiracle; si, sphenolateral; I, 

 trabecula; 2, optic nerve. 



Fig. 121. — Late chondrocranium of Ceratodus; B, hyoid and hyomandibula in later 

 stage (Sewertzoff, '02). b, basihyal; c, ceratobranchial; ct, trabecular cornu; ec, epi- 

 chordal cartilage; h, hyoid; he, hypochordal cartilage; hh, hypohyal; hm, hyomandibula; 

 hn, hyomandibular nerve; m, Meckelian; n, nasal capsule; nc, notochord; oc. otic cap- 

 sule; pb, palatobasal process; q, quadrate; s, nasal septum; si, sphenolateral region; /, 

 trabecula; v, vertebrse; II-IX, nerve exits. 



from this region on either side, the lower comparable to the cornua, the upper 

 forming the roof of the nasal capsule. The cranium is platybasic, the hypo- 

 physial fenestra large. 



