SU15-CLASS I 



SELACHII 



29 



arched, without lateral denticles, and longitudinally keeled ; all superfi 



marked by reticulating ridges. Notochord persistent. Head spines 



Hyhodus and Acrodus. Dorsal fin spines marked 



by stellate tubercles, sometimes partly fused 



into longitudinal ribs ; also bearing a median 



longitudinal double series of tubercles on their 



hinder face. Middle and Upper Jurassic. A. 



ornatissimiis, Ag. (Strophodus reticulatus, Ag.), 



(Fig. 54), well known from Corallian, Oxfordian, 



and Kimmeridgian of Europe. 



Palaeospinax, Egerton. Dentition compara- 

 tively specialised ; the few anterior teeth high- 

 crowned and prehensile, with a single pair of 

 lateral denticles; posterior teeth low-crowned, 

 with two or three pairs of lateral denticles 

 reduced to minute beads. Yertebrae very 

 slightly asterospondylic. Dorsal fin spines 

 smooth and enamelled, without posterior 

 denticles. Shagreen fine and dense; no head 

 spines. P. priscus, Egert., known by nearly 

 complete specimens from Lower Lias, Lyme 

 Eegis. Fragments of other species from Upper 

 Lias, Würtemberg. 



cially 

 as in 



Fig. 54. 



Tootli of Asterucanthns (Strophodus reticulatus, 

 Ag.), inside view aud upper view. Corallian ; 

 Tonnere, Yoiiiie. 



Fig. 53. 



Asteracanthus ornatissimus, Ag. Doixal 

 fin-spine, lateral view (A), posterior view 

 (jB), aiul detached tubercle eularged (('). 

 Portlandiau ; Solenre, Switzerlaiid. 



Synechodus, Smith Wood ward (Fig. 55). Almost identical with Palaeo- 

 spinax, but teeth with more numerous lateral denticles, and vertebrae more 

 distinctly asterospondylic. Complete jaw of S. duhrisiensis, Mackie sp. (Fig. 

 55), and greater part of skeleton of same species known from the Lower 

 Chalk of England. Teeth of other species from various Cretaceous formations 

 in Europe, Cretaceo-Tertiary of New Zealand and Patagonia, and one (S. clarki, 

 Eastman) from the Maryland Eocene. 



Cestracion, Cuv. (Heterodontus, Blainv.; Drepanephorus, Egert.), (Figs. 56, 

 57). Symphysial teeth small, numerous and prehensile, with a median cusp 

 and one pair of lateral denticles; lateral teeth in oblique series, with faintly 

 keeled and fiiiely rugose crown. Vertebrae asterospondylic. Dorsal fin 



