64 
WOODWARD : SHARKS OF THE CRETACEOUS PERIOD. 
Some of the finest fossil Elasmobranch heads yet discovered 
constitute the material on which this species is based, and the generic 
determination has been arrived at solely from the evidence of the 
teeth and associated ribbed dorsal fin-spines. Two other essential 
characters must be known before it is definitely proved that such a 
determination is correct, and they are the characters revealed by the 
new specimens now under consideration. 
The fossils in question were lately obtained by the British 
Museum from the collection of the late Mr. S. H. Beckles, F.R.S., 
of St. Leonard's-on-Sea. They comprise numerous skulls and two 
portions of the trunk of the typical Hybodus basanus from the 
Wealden of the Sussex Coast, probably from Pevensey Bay. All the 
remains of heads require much extrication from matrix before they 
exhibit the various cartilages and teeth, but the portions of trunk 
are already as completely exposed as possible. 
The two essential characters concerning which these speci- 
mens afford information are (i.) the presence of hooked cephalic 
spines, and (ii.) the occurrence of a vacant space between the neural 
and haemal arches of the endo-skeleton, which must have been 
occupied by a persistent notochord. Moreover, the fossil shown 
in pi. I., fig. 1, seems to exhibit for the first time in Hybodus the 
great triangular basal cartilage by which each dorsal fin-spine is 
supported. 
It is worthy of note that among the numerous heads of H. 
basanus at present in the British Museum, only a very small pro- 
portion display the cephalic spines. Indeed, as the same remark 
applies to the large series of known specimens of the typical species 
from the Lias, the writer is much inclined to think that the presence 
or absence of these spines will eventually prove to be a sexual 
character. However that may be, five or six of the Wealden skulls 
{e.g., number P. 6356) exhibit the characteristic hooks in their usual 
position behind and above the orbit, and these are no smaller in size 
than those of the typical H. reticulatus. It is only to be observed 
that whereas in the earlier species the spines invariably occur in two 
pairs, there is no certain evidence of more than one pair in the fossils 
from the Wealden. 
