66 
WOODWARD : SHARKS OF THE CRETACEOUS PERIOD. 
the articulation of the pterygo-quadrate cartilage with the postorbital 
region of the cranium, and the markedly asterospondylic character of 
its vertebrae. Like Palceospinax and Cestracion, this shark must 
have been destitute of cephalic spines ; and there can be no doubt 
that the dorsal fin-spines were smooth, without posterior denticles. 
In connection with the striking resemblance between the Liassic 
and Cretaceous genera thus compared, a detached dorsal fin-spine 
from the Gault of Folkestone, recently presented by Mr. J. T. Day, 
F.G.S., to the British Museum, is worthy of special remark. This 
fossil is shown of the natural size in pi. 11., fig. 2, and is precisely 
similar in form to that of the recent Cestracion and several Spinacidse. 
Instead, however, of its inserted portion being coated by an even film 
of ganoine, the shining layer towards the base of insertion becomes 
subdivided into a cluster of rounded tubercles. This is a condition 
not infrequently met with in the spines of Palceospinax, but un- 
known, so far as the writer is aware, in the recent or extinct species 
of Cestracion. The new fossil may thus be referred to Synechodus 
with much probability of correctness, and in that case makes known 
another slight point of resemblance between this genus and its fore- 
runner of the Lias. 
Synechodus illingworthi (Dixon). 
1850. Acrodus illingworthi, F. Dixon. Geol. and Foss. Sussex, 
p. 364, pi. 30, figs. 11-12, pi. 32, fig. 9. 
1887-89. Acrodus (?) illingworthi, A. S. Woodward. Geol. Mag. [3], 
vol. iv., p. 104 ; Proc. Geol. Assoc., vol. x., p. 290 ; Cat. 
Foss. Fishes Brit. Mus., pt. i., p. 297. 
1889. Hybodus (?) sp., A. S. Woodward. Cat. Foss. Fishes Brit, 
Mus., pt. i., p. 277. 
A recent examination of the Upper Cretaceous teeth named 
Acrodus illingworthi, in company with Prof. H. G. Seeley, has con- 
vinced the present writer that they must be referred to the hinder 
part of the dentition of a large species of Synechodus. To the middle 
of each ramus of the jaw of the same species may also be assigned 
the small group of Hybodont teeth from the English Chalk entered 
in the British Museum Catalogue as very doubtfully pertaining to 
Hybodus (number 45,311). Some of the detached hinder teeth are 
