PURBECK FORMATIONS. 85 



posterior border is applied to the next tooth {p 4), the anterior angle of which is reached 

 by the apex of the present tooth. The upper and anterior border of the rhomb forms 

 the free trenchant part of the crown and slopes down to the angle of the antecedent 

 tooth (f» 2). This border presents four dentations, from which as many ridges are con- 

 tinued downward and forward, but subside before they reach the middle broadest part 

 of the crown. The beginning of the posterior root is shown. This tooth is less than its 

 homologue in VI. BecMesii (fig. 1\ a, p 3). 



The last molar (fig. 16 a, jo 4) presents the usual subquadrate form, with the hinder 

 root the largest. Eight denticles arm the trenchant border, from which the ridges traverse 

 in the usual oblique course the upper half of the crown. There is a slight protuberance 

 above each root, the rest of the crown being flat. A minute short vertical ridge is on the 

 fore half of the crown, below the oblique ridges ; and a similar short vertical ridge may be 

 discerned near the hind part of the penultimate molar (jo 3), recalling the structure seen 

 in Plagiaulax minor ; and of which there is no trace in PL BecMesii. An impression of 

 the bituberculate inner wall of the crown of a small molar {m 1) is visible on the matrix 

 behind the last premolar ; it is followed by the impression and also part of the inner wall 

 of a second molar (m 2). This appears to be more even in character than the first ; it is not 

 developed into two tubercles so large and well marked ; it is of the same, generically, 

 small size. Beyond the last molar rises the impression of part of the beginning of the 

 coronoid process (^). 



Dividing the dental series of this specimen from the tip of the incisor to the back part 

 of the last molar into fifteen parts, the incisor occupies five parts, the premolars five parts, 

 the diastema two parts, and the molars {m 1 and 2) three parts. 



If the only differences between the present specimen and the type of Plagiaulax 

 BecMesii had been in the less robust mandible and more slender incisor, such characters 

 might have been interpreted as sexual ; but the difierence of the configuration of the 

 outer surface of the jaw, and more especially in the proportions of the premolars, point to 

 a derivative stage which would support a distinct specific entry in Zoological Cata- 

 logues. It is instructive to discern in the differences supporting a Plagiaulax Falconeri 

 evidence of an intermediate position between PL BecMesii and PL minor. In size the 

 present species, which I dedicate, as of right, to the Founder of this most singular and 

 interesting genus, is the same or but very little inferior to the type of PL BecMesii. 



The specimen (fig. 16) does not appear to have come under the notice of Dr. Falconer. 



Plagiaulax medius, Owen. (Cut, fig. 7, p. 86, and fig. 12, p. 90, magn. 4 diam., 



after Falconer.) 



The subject of fig. 14, p. 289, of Falconer's ' Original Memoir' (loc. cit.), I have failed 

 to find after rigorous and reiterated search in the series of specimens of the Becklesian 

 Collection transmitted to me for description. It was not in that portion including the 



