746 MR. c. TATE REGAN ON [June 19, 



Family 5. OARCHARiiDiE, 



A nictitating membrane ; no oro-nasal grooves ; last one or two 

 gill-clefts above the base of the pectoral. Rostral cartilages 

 convergent anteriorly. Vei'tebral centra with secondary calcifi- 

 cations, starting near the central double cones and extending to 

 the periphery, forming four principal calcified a,reas (in the shape 

 of a Maltese cross), between which four uncalcified areas radiate 

 to the bases of the neural and hasmal arches ; from the central 

 double cones four calcified rays extend a greater or less distance 

 into the uncalcified areas. Pectoral fin with small propterygium 

 and mesopterygium, most of the x-adials being attached to the 

 metapterygium ; radials usually formed of three segments. 

 Mixopterygia with the free edges of the marginal cartilages not 

 coalescent. 



The first dorsal is usually in advance of the ventrals, rarely 

 {Tricenodon) partly above them. The spiracles are small or absent. 

 The caudal fin is strongly heterocercal in the pelagic genera, but 

 not in the others. The family appears to date from the Eocene 

 and there are no extinct genera. The Hammer-headed Sharks 

 {Sphyrna) perhaps deserve to i-ank as a subfamily {Sphyvnince). 



Division 3. Squaloidei. 



Five or six gill-clefts on each side ; two dorsal fins ; in the 

 living forms each dorsal fin preceded by a spine or the anal fin 

 absent. 



The reasons which induce me to include the Cestraciontidse and 

 their extinct allies in the same suborder as the Sqvialidse are 

 especially derived from the structure of the median and paired 

 fins and of the mixopterygia, which affords sufiicient evidence of 

 the close relationship of these Sharks. Each of the families 

 defined below possesses certain features of specialisation and, with 

 the exception of the Squatinidfe, which are modified Squalidse, 

 must be regarded as having evolved along divergent lines from 

 the same ancestral stock, Avhich the Squalidte, although the anal 

 fin is absent, resemble perhaps more than the Oestraciontides. 



The rostrum is typically a simple prolongation of the anterior 

 wall of the cranium (text-fig. 118, A, p. 742), and never has the 

 form characteristic of the Galeoidei. The pterygo-quadrate is 

 either not articulated to the cranium (Squalidse, Squatinidse), or 

 it may have acquired a prasorbital (Cestraciontidse) or postorbital 

 (Hybodontidse) articulation. 



In the Squalidae, Squatinidse, and Cestraciontidse, the primary 

 calcifications of the vertebral column are in the form of double 

 cones which constrict the notochord vertebrally. In the first 

 family secondaiy calcification is absent, except in Pr%stiophor%is^ 

 which has a calcified ring external to and separated from the 

 •central double cone. In Squatina a series of concentric calcified 

 laminae stirrounds the central double cone. In the Cestraciontidse 



