24 FAUNA OF THE CORNBRASH. 



1880. Pliosaurus ferox, Lydekker, Geol. Mag. [3], vol. v, p. 353; and Cat. Brit. Foss. Rept. B. M., 



vol. ii, p. 145. 

 1890. Pliosaurus ferox, Lydekker, Quart. Journ. Geol. Soc, vol. xlvi, p. 50, pi. v. 



Skiatype of species. 1 — Described thus, /. c. : " At the base of the enamel the 

 two diameters of the tooth are as 15 to 13. The root is entirely smooth and 

 conically hollowed at the base. The enamelled portion is rather strongly curved 

 and divided into two parts by a ridge extending to the summit on the anterior and 

 posterior diameters. A strong ridge is also seen on each of the lateral faces, which 

 continues the whole length of the enamel. The external face is divided into 

 two by a strong keel reaching the summit ; otherwise it is almost quite smooth, 

 Elsewhere there are strong keels on the inner side, reaching various distances 

 towards the summit." From the lowest zone of the Oxford Clay of Le Wast, 

 near Boulogne. 



Description. — Two teeth from the Cornbrash of Rushden showing the charac- 

 teristic matrix are referred to this species. They do not agree absolutely with the 

 above type, but more closely resemble specimens from the Oxford Clay of Peter- 

 borough which have been referred to it, being there associated with more typical 

 forms. In the specimen figured in PI. I, fig. 1 the section at the base of the 

 crown is nearly circular, and the conical form is very regular; the fine ridges of 

 the enamel proceed directly towards the summit, without showing the twist seen 

 in more typical forms. The longitudinally convex side has very few ridges (7), 

 and these not continuous. It is bounded by a continuous ridge on each side. 

 The concave part has twenty-four more ridges of various lengths, but none 

 especially central as in the type, and all are weak, giving a smooth aspect to the 

 whole. In PL I, fig. 2, the ridges are somewhat stronger and more numerous on 

 both sides but have no twist. 



As we have only the teeth for examination, they are not sufficient for dis- 

 cussing the genus of the whole animal, for which reason a temporary name, 

 Lioplewrodon, was given them by the author ; at a later date he decided it was a 

 Polyptychodon. The teeth certainly agree better with Owen's definition of the 

 latter genus, and disagree with his definition of Pliosaurus, so that as far as the 

 teeth are a guide the name last adopted by Sauvage should be right. The asso- 

 ciated bones, however, in this country have led to their being placed in Pliosaurus* 

 If then the bones do not justify the use of the name Polyptychodon, neither do the 

 1 If the original definition is defective or deals with a part only, the specimen is what may be 

 called a skiatype (shadow-type), subject to the laws of priority, but if it be erroneous in any essential 

 respect it ceases to be the definition of a real species at all, and by the same laws is barred from use. 



Type-specimens being the only ultimate units of fact, the union of associated specimens with them 

 into a species and the union of several such species into a genus are matters of more or less certain 

 inference only, which will naturally vary. Hence the bulk of the synonymy, long lists of which seem 

 to me to be often little more than the substitution of one man's inferences for another's without 

 reasons assigned. 



