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Bashford Dean Memorial Volume 



Text-figure 31. 

 Dentition of Synechodus dubrisiensis Mackie, a member of the family Heterodontidae (Cestraciontidae) 

 represented only by fossils. These teeth are twice natural size, with six separate teeth enlarged four times. 



Upper Cretaceous, Sussex. 

 After Woodward, 1889, Part 1, Text-fig. 12, p. 326. 



except that the anterior teeth of Synechodus are larger than the posterior ones. The teeth 

 of Paleospinax show progress in the direction taken by Heterodontus: the few anterior 

 teeth are high'crowned and prehensile with only a single pair of lateral denticles, while 

 the posterior teeth are low-crowned with two or three pairs of lateral denticles reduced to 

 minute beads (Zittel, 1932). Finally in Heterodontus, the only genus represented by 

 living specimens, the anterior teeth of the adult are typically tricuspid, the central cusp 

 predominating; while the posterior teeth are large, and set in oblique rows, without 

 cusps but with the grinding surface of each tooth traversed by a slender longitudinal 

 ridge — unless this is worn away by use. Nearly complete skeletons of Heterodontus 

 (Cestracion) have been found in the Lithographic Limestone (Upper Jurassic) of Bavaria 

 and the Chalk of England. The teeth of these fossils, which include several extinct 

 species, are said to differ little from those of recent examples of the genus save that the 

 crowns of the grinding teeth are rugose in addition to having a longitudinal keel. 



The spines of the dorsal fins are not limited to sharks of the families under consider' 

 ation, but one of the most obvious differences between the two families is the ornamenta' 

 tion of the dorsal spines in the Hybodontidae and the almost entire lack of it in the 

 Heterodontidae. The "ornamentation" consists of longitudinal ridges along the sides and 

 sometimes the front of the spine, and the presence of tubercles on its rear surface. In two 

 genera of fossil Heterodontidae, Paleospinax and Synechodus, the dorsal fin spines are 

 almost uniformly smooth, and in Heterodontus they are entirely smooth. 



