Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/313

MASON—DAKOSAURUS. 219 It is noticed by Prof. Owen*, who rightly includes it among his "amphicoelian" Crocodilia.

Teeth of Dakosaurus are said to occur, associated with remains of Pliosaurus, Gyrodus, Spoerodus gigas, &c., in the Potton Sands described by Mr. Seeley; these are its companions, both in the Upper White Jura of Schnaitheim and in the Kimmeridge Clay of England. Mr. Walker† also mentions it as occurring in beds probably of the same age at Upware, in the Fens of Cambridge, and in similar company.

I have examined, through the kindness of Mr. W. Davies, a suite of the Potton fossils at the British Museum; but it would be almost impossible, owing to the worn condition of these fossils, which have evidently been derived from older rocks, to determine with any degree of certainty its presence among them.

The British Museum contains numerous specimens of Dakosaurus from the German locality, as well as a single tooth from the Kimmeridge Clay of Boulogne and one from that of Ely, near Cambridge. A comparison of these with the teeth from Oxford fails to discover differences of specific value; I therefore propose to retain M. Quenstedt's name of maximus for the latter.

Fig, 3. Fig. 4.

a. External view of tooth of Dakosaurus, the ridges forming the contour lines.

b. The same tooth, showing the pulp-cavity.

Side view of an anterior tooth of Dakosaurus.

This genus is characterized by large, conical, incurved and slightly recurved teeth, the smooth and polished enamelled crowns of which are traversed by two sharp, prominent, minutely crenulated longitudinal ridges, one forming the posterior, the other the anterior margin of the tooth; these ridges are situated "midway between the convex and concave curvatures of the tooth," which is compressed at the apex so as to give in transverse section a slightly oval figure pointed before and behind; but this character dies away towards the base, where the crown becomes cylindrical and ceases to be interrupted by the ridges; one of these, the anterior, is lost sooner than the other, which is continued a short distance into the cement-


 * Palaeontology, 2nd edit, p 300.

† Geol. Mag. vol iv. p. 309.