MACKIE — 0~S A NEW SPECIES OE HTBODUS. 



In all these 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 Hybodics, 

 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 A.crodus 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 Hybodus, 

 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 

 Clin", between Dover and Folkestone, this is very prettily shown ; 

 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. Hybodus 

 seems to us to make its first appearance, and in quantity, in the Lias 

 bone-bed. In Morris's " Catalogue" H. laviuscidus, H. minor, and 

 II. plicatilis are all referred, with a query, to the Trias, — the first 

 coming from Aust Cliff, the second having the localities Aust and 

 A x mouth, and the last that of Axmouth, recorded, seemingly, on the 

 verbal statement of Sir Philip Egerton. The H. keuperianus, en- 

 tered as from the Keuper of Worcestershire, is evidently the spine 

 of Nemacanthus, an Aust Cliff bone-bed fish. 



Of the first, II. 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 II. 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 Kegis, and they are ac- 

 companied in this locality by a kind of teeth very different from 

 those of Dorsetshire." The Muschelkalkj Hyhodus plicatilis, Agassiz 



