PURBECK FORMATIONS. 75 



§ XVI. Gt'Hus — Plagiaulax, Falconer, 1857.' 

 Species 1. — Plagiaulax minor, Falconer? Plate III, figs. 9, 9 a, 9 b. 



This species is represented by the dentigerous portion of the right mandibular ramus 

 with the teeth in situ (PI. IV, fig. 9, nat. size ; 9 a, magn. 3 diam. ; b, molars magn. 

 6 diam.). The lower border of the hind part with the ascending ramus is broken away. 



The teeth are the incisor {i), four premolars {p 4—2), and two molars [m i, 2). The 

 incisor is long, large, laniariform, more rounded or convex externally than in the larger 

 species (see PI. IV, fig. 11, a, i); the anterior end is broken off, but the impression left on 

 the matrix shows it to have been pointed, and also indicates a shallow longitudinal groove 

 on the inner side of the tooth, nearer the fore than the hind border {a i'). The length of 

 the exposed part of the tooth or ' crown' equals the fore-and-aft extent of the entire pre- 

 molar series. The direction of the crown is upward and forward, at an angle of 120° with 

 the alveolar line of the succeeding teeth : the convexity of the antero-inferior border and 

 a slight concavity of the postero-superior border give the appearance of a curve in the 

 same direction. 



The first premolar abuts against the fore part of the second. It rises at a distance 

 from the incisor equal to the breadth of the base of that tooth. It is minute, with a 

 crown longer or higher than its fore-and-aft breadth. Slightly bulging at its base, above 

 the single root, it becomes flat externally with the summit obliquely truncate. 



The crown of the second premolar (jo 2), larger and more compressed, swells out rather 

 more abruptly above the fang, from which protuberance the crown expands and flattens as 

 it rises, and again contracts to abut upon the fore margin of the next tooth. The apex of 

 the crown of jo 2 is marked by three fine short ridges directed from before upward and 

 backward ; below these the enamel is smooth and flat to the bulging base. The height 

 of the crown is greater than its fore-and-aft breadth, the extreme of which is about half 

 way to the angular summit ; this shape is due to a truncation in the same direction as in 

 the first molar, viz. from above downward and forward, which gives to the crown a rhom- 

 boidal figure. 



The third premolar {p 3) attains greater breadth in proportion to its height, 

 though the latter dimension of the crown still predominates : the thickness of 



' "An abbreviation for ' Plagiaulacodon,' from xXayios, oblique, and auXa^, groove, having reference 

 to the diagonal grooving of the premolars,' see ' Description of Two Species of the Fossil Mammalian 

 Genus Plagiaulax from Purbeck ;' by Hugh Falconer, M.D., F.R.S., F.G.S., in 'Quarterly Journal of 

 the Geological Society of London,' vol. .xiii, p. 261. 



- lb., p. 264, and p. 281, fig. 15. 



