750 MR. C, TATE REGAN ON [June 19^ 



cartilages of the dorsal fin are very similar to those of the existing 

 Cestraciontidse and Squalidse. The pelvic fin is remarkable for 

 its primitive structure, only 6 basalia being fused to form a basi- 

 pterygium, the posterior 8 being distinct. If Campbell Brown's 

 interpretation of the pectoral fin be accepted, its structure is quite 

 unlike that of any knowii Shark. I am convinced, however, that 

 he has mistaken the propterygium for the metapterygium and 

 vice versa, and that the fin is in reality almost exactly similar to 

 that of Squahis or Fristiophorus. 



B. Palceospinacince. Dorsal fin-spines smooth. No cephalic 

 spines. Vertebral column with calcified centra. 



The Triassic and Cretaceous genera Palceospinax and Synechodus 

 have a dentition not unlike that of Hyhodus, to which they are 

 evidently related. The postorbital articulation of the pterygo- 

 quadrate has been described in Synechodus by Smith Woodward. 



Family 3. Cestraciontid^. 



Body not depressed ; five gill-clefts on each side, the posterior 

 ones aloove the base of the pectoral fin. Paired oro-nasal grooves 

 present. Each dorsal fin preceded by a spine ; anal fin present ; 

 pectorals normally shaped. Pterygo-quadrate with prteorbital 

 articulation to the cranium, the palato-basal process being broad 

 and greatly developed. Vertebral centra calcified. 



The genus Cestracio7i, ranging from the Jurassic to the present 

 day, may be regarded as allied to the Hybodontidse in so far as 

 both have been derived from a common ancestor with a generalised 

 dentition and without articulation of the pterygo-quadrate to the 

 cranium. The resemblance of the lateral teeth to those of Acrodus 

 most certainly does not indicate any special relationship to that 

 genus, which is clearly a modified Hyhodus. 



Family 4. Squalid^. 



Body not depressed. Five or six gill-clefts on each side, the 

 last in front of the base of the pectoral, which is normally shaped. 

 No oro-nasal grooves. Each dorsal fin often preceded by a spine ; 

 no anal fin. Teeth small or moderate, sometimes conical or 

 cuspidate, often compressed. Pterygo-quadrate not articulated to 

 the cranium. Vertebral centra calcified. 



Of the Squalinee, Squalus and Centrophorus appear to date from 

 the Cretaceous. 



Some authors would place the genera with normal snout and 

 withovit apparent dorsal fin-spines in a distinct family, but it 

 must be borne in mind that the recent researches of Helbing have 

 shown that rudimentaiy spines are present. Moreover, in some 

 species of Gentroscymnus the spines are very small and do not. 

 even pierce the skin. 



Jaekel has described the anatomy of Fristiophorus in detail and 



