1906.] SELACHIAN FISHES. 749 



This type of fin is found in Squalus, Fristiojjhorits, and Hyhodus. 

 Modifications arise from the fusion of pi-o- and meso-pterygia 

 [Cestracion), or of meso- and meta-iDterygia {Ce7itrophorus), or of 

 all three {Scymnorhinus), and also from fusion of the proximal 

 segments of the mesopterygial radials {Cestracion). The pectoral 

 fin in Squatina is very similar to that of Squalus, but the pro- 

 pterygium is directed forward and bears several radials. 



The mixopterygia (text-fig. 117, A, p. 739) present certain con- 

 stant characteristics. The axial cartilage is cylindrical and distally 

 pointed ; it is separated from the basipterygium by one, two, or 

 three proximal segments ; the accessory cartilage, when present, 

 retains the form of a radial with but little modification. Usually 

 the ventral marginal cartilage is confined to the distal part of the 

 appendage, but sometimes (e. g. in Scymnorhinus) it extends nearly 

 to the proximal end of the axial cartilage. The glandular sac 

 contains no special glandular body, except in Squatina, which is 

 peculiar also in that a ventral covering piece is developed. 



Family 1, Cochliodontid^. 



Carboniferous Shai-ks in which " at least one of the transverse 

 -series of teeth encircling each ramus of the jaw is fused into a 

 continuous curved plate." Dorsal fin-spines often present ; 

 vertebral column uncalcified and probably acentrous. 



Family 2. Hybodontid^. 



Body not depressed ; five gill-clefts on each side. Each dorsal 

 fin preceded by a spine ; anal fin present ; pectorals normally 

 shaped. Teeth aU separate. Ptery go -quadrate with a well- 

 developed postorbital articulation with the cranium. 



Palpeozoic and Mesozoic Sharks v/hich may be grouped into two 

 subfamilies :^ 



A. RyhodontincB. Fin-spines with longitudinal lidges or series 

 of tubercles ; vertebral column uncalcified and probably 

 acentrous. 



This subfamily perhaps includes two groups which cannot, at 

 present, be properly defined. The Permian and Carboniferous 

 Orodus, Campodus, etc. are, according to Eastman, characterised 

 by the presence of a single enlarged series of symphysial teeth, 

 presumably belonging to the lower jaw. The Mesozoic Hyhodus, 

 Acr edits, and Asteracanthus form a very natural group ; the 

 males have paired postorbital cephalic spines. The lateral teeth 

 are elongate and the symphysial teeth few and large (in comparison 

 with Cestracion). From Hyhodus, with conical or cuspidate teeth, 

 we pass to Acrodus, with rounded non-cuspidate teeth, and thence 

 to Asteracanthus, with flattened quadrate teeth. Extraordinarily 

 Avell- preserved remains of Hyhodus have been described by Campbell 

 Brown. They show the postorbital articulation of the pterygo- 

 quadrate and the structure of the median and paired fins. The 



