PURBECK FORMATIONS. 79 



prominence (fig. 11 k, b) a, little below the hind part of the socket of the last pre- 

 molar leads backward to the broken origin of the outer part of the anterior border of the 

 base of the coronoid process, and a feebler bulge or vertical convexity of the line extends 

 forward below the premolar alveoli to near the outlet of the socket of the incisor. The 

 concavity {a), deepest at the back part of this portion of jaw, gradually shallows and con- 

 tracts to end below the alveolus of /; 2. The second vertical convexity {c) of the outer 

 surface of the ramus, forming the lower boundary of the concavity [a) is continued into 

 the thick rounded under border of the jaw. 



The ramus loses depth as it advances and terminates anterioily in the socket of the 

 large subcompressed laniariform incisor (fig. 11, i). The crown of this formidable tooth 

 is directed upward at an angle of 110° with the line of the premolar alveoli; the tooth 

 as it advances contracts or loses fore-and-aft breadth, and is slightly curved upward 

 to its pointed extremity. The transverse breadth, which is small, is maintained nearly to 

 the apex. The outer side of the basal third shows a shallow medial longitudinal impres- 

 sion, contracting to lose itself in the uniform subconvex level of the rest of the outer 

 surface. This depression recalls that at the corresponding part of the base of the canine in 

 Triacantliodon (jy, PI. IV, fig. 8 A, c). The anterior border of the incisor is blunt to near 

 the apex ; the posterior border shows a narrow flat tract with sharp margins ; the entire 

 crown is invested by enamel. It is implanted, as far as the socket is exposed, by a single 

 cement-clad root, which slightly contracts as it sinks into the bone. 



The incisor is relatively as long as in Plagiaiilax minor (PL IV, fig. 9), but is broader 

 antero-posteriorly, less convex, and less uniformly so externally, more recui'ved toward the 

 apex ; in other words, more fitted for piercing as might become a larger and stronger 

 species. 



The three compressed lamelliform premolars progressively increase in height and, in a 

 greater degree, in fore-and-aft extent, as they recede in position. The crowns of the three 

 teeth (/? 2, 3, 4) are in very close contact, and are so coadjusted as to give to their tren- 

 chant borders a continuous convex outline, acting as one great carnassial. 



The first premolar (fig. 11 a,/; 2) is the least. It arises about twice its own breadth 

 from the outlet of the incisive alveolus, leaving a distance of that extent between it and 

 the incisor. The height of the crown is twice its breadth. The fore part of the base 

 swells into a smooth convexity. The contracting summit bends slightly back and abuts 

 against the middle of the fore part of the second premolar. The outer side of the upper 

 part of the crown of the premolar {p 2) shows an oblique ridge. It appears to be 

 implanted by two fangs, an alveolar process rising to their interspace. 



The second premolar {p 3) is implanted by a large anterior and a small posterior fang. 

 The outer side of the anterior root swells out at the base of the crown into a smooth convex 

 protuberance. The outer alveolar wall rises in an angular form between this and the more 

 inwardly inserted small hind root. The crown, narrow transversely, expands from before 

 backward, and then contracts to a subconvex trenchant border. Of this the cutting power 



