62 
THE HYBODONT AND CESTRACIONT SHARKS OF THE CRETACEOUS PERIOD. 
BY A. SMITH WOODWARD, F.G.S. 
Plates I. and II. 
The fish fauna of the Cretaceous period is of most interest as 
exhibiting the last survivors of many of the ancient Mesozoic types 
and the dawn of the great host of modern forms which characterise 
the Tertiary epoch. Among sharks the Hybodonts and Cestracionts 
are becoming extinct, Cestracion itself alone passing into the Tertiary 
beyond, while numerous representatives of the characteristically 
modern group of Lamnidse suddenly make their appearance in the 
Gault, Greensand, and Chalk. Among other fishes the same kind 
of change is apparent, and it remains for future discoveries of inter- 
mediate forms to indicate the precise lines of evolution of which only 
widely separated terms are as yet well known. 
The surviving Jurassic families and genera that become almost 
or quite extinct in the course of the Cretaceous period are thus 
worthy of special attention. It is instructive to compare them 
with their early representatives at a time when such forms were a 
dominant type of life. The materials are often unfortunately 
fragmentary, but many noteworthy facts are discoverable in the 
attempt at comparison, and it is the object of the following notes 
to deal with some of the Cretaceous Hybodont and Cestraciont 
fossils from this point of view. 
Genus Hybodus. 
[Agassiz, Poiss. Foss., vol. iii., 1837, p. 41 ; A. S. Woodw r ard, Cat. 
Foss. Fishes, Brit. Mus., pt. i., 1889, p. 250.] 
As represented by its typical species in the Lias, the principal 
characters of the skeleton of the genus Hybodus are now well-known. 
The paired fins and the anal and caudal fins alone remain undiscovered. 
In shape the fish seems to have much resembled the existing Port 
Jackson Shark {Cestracion) or the typical modern Lamnidae ; and the 
presence of two dorsal fins is proved by the frequent occurrence of 
the two spines with which they were armed. The mouth is deeply 
