CERATODUS 83 



outside the outer lining of the coelom. From their position 

 they are known as " ventral " or pleural ribs. 



Limbs and Girdles. — The primitive pectoral girdle is 

 cartilaginous and composed of paired dorsal scapular regions, 

 and ventral coracoid regions, which latter are joined to one 

 another in the midventral line. Overlying this are the 

 membrane-bones, clavicle, clei thrum, and the post- temporal 

 which connects the girdle with the hinder part of the skull. 



The pelvic girdle is formed of a median Y-shaped cartilage 

 with the prongs directed backwards and articulating with the 

 pelvic fins. 



The fins are covered with scales. Their endoskeleton is 



pr. 



Fig. 40. — Ceratodus : skeleton of the pectoral fin, showing the " archiptery- 

 gial " structure, with an axis (a), bearing preaxial (pr) and postaxial 

 (po) radials. 



cartilaginous and composed of a long central axis of about 

 twenty pieces, tapering away to the tip. On each side of this 

 axis are radials (pre- and postaxial). Beneath the scales are 

 the camptotrichia. 



Teeth. — The plates of teeth, which are firmly attached to 

 the prevomer, pterygo-palatines, and splenials, are the result 

 of fusion of separate teeth. 



Alimentary Canal. — In its main lines the alimentary canal 

 does not differ much from that of Scyllium, with a spiral valve 

 in the intestine. Its most interesting and important feature 

 is that in the floor of the oesophagus there is an opening (the 

 glottis) leading to a tube or trachea which passes up round the 

 right side of the gut to the lung. This is a large sac with 



