44 PISCES. 



into one continuous piece. The coalescence of the teeth is 

 so complete in all cases, that their mode of growth is much 

 changed ; the once-separate outer tooth can no longer fall 

 away from the mouth as growth of tooth-substance takes 

 place within, and the external border thus coils inwards like 

 a scroll, as shown in transverse section (fig. 33, 8). The 

 compound crushing plates of the Cochliodonts hence result 

 not only from the fusion of adjacent teeth, but also from 

 the coalescence of several successional teeth. The typical 

 Cochliodonts are exclusively Carboniferous, and it is note- 

 worthy that the least specialized form of dentition (Pleuroplax) 

 has the most extensive range, being found both in the lower- 

 most and in the uppermost strata of that period. 



The Cestraciontidae, though now on the verge of extinc- 

 tion, were the most characteristic and abundant sharks of the 

 Mesozoic Era, and it is extremely probable that Orodus, Sphe- 

 nacanthus, Tristychius (Carboniferous), and Wodnika (Permian) 

 may also be placed here. In all the satisfactorily-known genera 

 each of the dorsal fins is armed with a spine, the first opposite 

 to the space between the pectoral and pelvic fins. The teeth 

 are mostly obtuse, never fused into continuous plates ; and 

 several series are simultaneously in function. So far as is known, 

 the upper element of the hyoid arch in this family does not 

 adequately perform the normal office of a hyomandibular ; but 

 it is noteworthy that the secondary direct articulation of the 

 pterygo-quadrate arcade with the cranium is not always pre- 

 orbital as it is in the existing Gestracion. In the Cretaceous 

 genus Synechodus, the facette is post-orbital, exactly as in the 

 Notidanidse. The nearly complete skeleton of a shark hitherto 

 not satisfactorily distinguished from Cestracion has been found 

 in the Lithographic Stone (Upper Jurassic) of Bavaria (C. 

 falcifer), and characteristic teeth occur in nearly all subsequent 

 formations. The best-known extinct genera are Hybodus, 

 Palceospinax, and Asteracanthus. 



Hybodus (figs. 34, 35). The teeth are like those of Synechodus (fig. 

 36), conical or cuspidate with more or less striated crown, and there is 

 one principal coronal elevation with one or more lateral prominences on 

 either side diminishing outwards; the root or base is depressed. The 



