MACKIE — ON A NEW SPECIES OF HTBODUS. 
243 
In all tbese teeth the central tubercle is much elongated, and in the 
front teeth assumes a regular tooth-like form, with a less expanded 
base, apparently only about equal in length to the height of the 
principal central tubercle. As, however, the teeth recede towards 
the angle of the jaw, the central tubercle becomes more and more 
depressed and the base more and more expanded ; the central tubercle 
at last becoming very little elevated, and the lateral denticles very 
numerous, but with elevations gradually lessening towards the outer 
ends of the base, in the invariable manner of the teeth of Hyhodus, 
and which to us forms the distinguishing feature of the teeth of that 
genus from the teeth of species of Cladodus, in which, on the con- 
trary, the denticles increase in size as they recede from the central 
tubercle. In all else the two genera seem very closely similar. The 
teeth of Acrodus are also very close in their characters. 
In these remarks, it will be seen that we differ from Sir Philip 
Egerton as to the general uniformity of the teeth of Hyhodus, 
and which, on the contrary, we believe varied, as sharks' teeth are 
well known to do, according to their positions in the jaw. In the 
specimen we are noticing, from the grey chalk of, probably, Abbot's 
Cliff, between Dover and Folkestone, this is very prettily sliown ; 
and the little group of four or five back teeth (seen in our Fig. 1, 
PI. XIII.) exhibit this feature in a remarkable and exquisite manner. 
We are inclined, too, to raise a question as to the range in geo-. 
logical time of the genus, at least within the British area. Hyhodus 
seems to us to make its first appearance, and in quantity, in the Lias 
bone-bed. In Morris's "Catalogue" H. Iceviusculus, H. minor, and 
R. plicatilis are all referred, with a query, to the Trias, — the first 
coming from Aust Cliff, the second having the localities Aust and 
Axmouth, and the last that of Axmouth, recorded, seemingly, on the 
verbal statement of Sir Philip Egerton. The H. Tceuperianus, en- 
tered as from the Keuper of Worcestershire, is evidently the spine 
of Nemacanthus, an Aust Cliff bone-bed fish. 
Of the first, //. Iceviusculus, however, Agassiz, who described it^ 
mentions it as " a very little fragment of a ray of this species in the 
Museum of Bristol, coming from the Lias of Aust Cliff." Of the second 
also, he says, " The rays designated under the name of H. minor, are 
really only found in the Lias of the environs of Bristol at Aust Cliff, 
where we do not find great rays as at Lyme Regis, and they are ac- 
companied in this locality by a kind of teeth very different from 
those of Dorsetshire." The Muschelkalk, Hyhodus plicatilis, Agassiz 
