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§ XV. Genus — Triacanthodon, 1 Owen. 

 Species 1 . — Triacanthodon serrula, 2 Ow. Plate IV, figs. 7, 7 a, 8, 8 a. 



This genus and species are exemplified in two slabs, counterparts of the same split 

 block of Purbeck Shale, with the left mandibular ramus of a young individual. 



One portion or slab (PI. IV, fig. 7, nat. size, and 7a, magn. 2 diam.) includes the 

 ascending ramus, the last molar tooth in its formative cell, and an impression of the rest 

 of the bone and teeth ; the other portion (fig. 8, nat. size, and 8a, magn. 2 diam.) includes 

 the part of the ramus and teeth anterior to the penultimate molar, and the impression of 

 the hinder part of the jaw with those of the last two molars. The portion of the ramus 

 including the penultimate molar, which intervened between the fractures in figs. 7 and 8, 

 had been lost before the specimen came into my hands ; it is well represented, however, 

 by the impressions of its outer and inner surfaces in the counterpart slabs. 



The fore part of the ramus in fig. 8 shows the outer surface, the hind part in the 

 opposite slab (fig. 7) shows the inner surface, of the mandible. 



In the fore part of the jaw the outer incisor (fig. 8a, i 3) is preserved ; it is small, 

 conical, thick, outwardly convex, with a sub-obtuse apex ; there is a feeble indication of 

 two shallow longitudinal linear impressions bounding a middle tract of the outer con- 

 vexity, near the base of the crown. The canine (ib. c) is long, large in proportion to the 

 incisor, strong, sub-recurved, sharp-pointed, with a longitudinal indent near the basal 

 part of the outer side of the crown, indicative of a tendency to division of the implanted 

 root. The canine follows the incisor without any diastema. The concave line of the 

 hind border of the crown is arrested three fourths of the way toward the base by a slight 

 hinder projection — a quasi feeble ' talon.' 



A very short interval divides the canine from the first premolar (p i). This is small, 

 two-rooted, sub-compressed, with a hind cusp almost equalling the main cone, and with a 

 small anterior basal cusp; the whole crown is low in proportion to its fore-and-aft 

 extent. 



The second premolar {p 2), similar in form to, but somewhat larger than, the first, 

 has a more elevated main or mid cone ; the front cusp or talon is rather more developed 

 than in p \. 



The third premolar (p 3) shows a markedly larger size, especially in the relative 

 height and breadth of the main cone. The anterior basal cusp is low ; the posterior cusp 



' rpe'n, three ; aKavda, spine; obovs, tooth. 



2 Serrula, little saw, suggested by the row of denticles formed by the five three-spined teeth. 



