PURBECK FORMATIONS. 77 



surface of the horizontal ramus is thence continued in a gentle curve below the alveoli of 

 the premolars, forward, to the upper part of the socket of the incisor ; the prominence is 

 not well defined, but it gives a vertical bulge or convexity to the outer side of the 

 jaw. 



The alveolar border rises in well-marked angles into the interspaces of the premolars, 

 or of their roots. 



The crown or exposed part of the incisor formed, when entire, two sixths of the 

 total extent of the dental series ; the premolars two fifths ; the molars 

 one fifth. Fig. 6. 



Fig. 9, PL IV, gives the natural size of the specimen. If the ascend- ^S^-^^rp^ 

 ing ramus of the jaw be restored after the type of that of the larger Mandible and 



tGctfi ndt siz6 



species of Plagiaidax (ib. fig. 10), the length of the lower jaw, including Piagiauiax mmor. 

 the incisor, would be seven and a half lines, as in the woodcut, fig. 6. 



Species 2. — Plagiaulax Becklesii, Falconer} Plate IV, figs. 10, 10 a, 10 b, 11, 11a. 



The type of this species is preserved in the counterpart slabs of a split block of 

 Purbeck shale, of which one contains the hind half of the right mandibular ramus, with 

 the impression of part of the fore half (PI. IV, fig. 10, nat. size; 10 a, magn. 3 diam.) ; 

 the other contains the fore half of the same ramus with the teeth and the impression of 

 the major part of the rest of the bone (ib. fig. 11, nat. size; 11 a magn. 3 diam.). The 

 inner side is exposed of the hind half, the outer side of the fore half. 



The fore half contains the incisor (fig. 11 h, i) and three premolars (ib.jo 2, 3, 4) ; 

 the hind half shows the shallow sockets of two small molars (fig. 10 a, ?m i, m 2). 



The condyle of the jaw (figs. 10 c, 10 b) is unusually large, especially in the vertical 

 direction. It extends to the lower border of the ramus, the angle — almost a right one — 

 being formed by their meeting or intersection at a. A narrow tract of fracture 

 indicates the homologue of the angular process to have been thence directly inflected as 

 a thin plate, the base of attachment of which was continued forward below the pterygoid 

 depression, to the entry of the dental canal {d). 



The condyle (c) is convex transversely and vertically, the articular surface curving 

 from before backward, downward, and again slightly forward, to the extent of nearly 

 a half circle : its breadth rapidly increases from the upper end to one third down, 

 then gradually decreases to near the angle. The smooth articular surface is best 

 marked upon (and was, perhaps, confined to) the upper broader part of the condyle, 

 the lower boundary, as in Tliylacinus, not being defined. The narrowing is chiefly from 

 the inner side; so that the outer contour of this vertical condyle (fig. 10 b, 0) is uniformly 



' Op. cit., pp. 262, 278, 279, figs. 1—5, 7—14. 



