PURBECK FORMATIONS. 83 



external!}^ and internally, and the proportion of the transverse basal breadth to the height 

 of the crown. In this section the tooth is spear-shaped. 



Externally as well as internally the lower swollen border of the crown is notched, indi- 

 cative of the division of the root, and the part in front of the notch is more prominent than 

 the one behind ; it is also lower placed. The -outer side of the crown (fig. 18 a, ^ 4) is flat 

 from the base to the cutting edge, and no indent marks the base of the crown as a cingulum. 



The small anterior premolar {p 2) repeats the character of the fore part of the succeed- 

 ing premolars, in the smooth enamelled tuberosity by which the crown rises from the root. 

 The narrow crown slopes backward, and is applied like a buttress against the fore part of 

 the next premolar. 



The grooves are limited to the upper half of the outer surface of the crown of^ 3 

 (fig. 13 a), but descend a little lower on the inner surface (fig. 13 b). The anterior den- 

 ticle of the serrate cutting edge bounds the foremost short groove ; the second ridge com- 

 mencing below the first denticle runs along a greater proportion of the cutting edge before 

 terminating in the second denticle. The third ridge beginning near the fore margin of 

 the crown extends obliquely upward and backward to the third denticle near the hind 

 part of the cutting edge. The fourth ridge terminates in the hind angle of that edge. 

 The groove below the fourth ridge is completed by a fifth, parallel with the others, and 

 terminating a little below the hinder angle of the crown. 



In the third or last premolar (/» 4) four grooves on the upper half of the outer surface 

 have directions nearly parallel with those of p 3. The foremost denticle at the anterior 

 angle of the crown bounds the first short groove. The four succeeding ridges all begin 

 at or close to the anterior margin of the crown. 



The accentuation of the outer surface of the fore part of the mandibular ranuis is well 

 shown in this fragment. A low ridge {t, fig. 13a), one fourth of the depth of the ramus 

 from the alveolar margins, bounds above a shallow depression at the middle third of that 

 surface which narrows to a point forward beneath the fore part of the first premolar {p 2). 

 Anterior to this point, or angular end of the depression, is a single well-defined circular 

 outlet of the dental canal [d). 



The incisor is subcompressed. The section of the base of the exserted crown is a 

 long ellipse with the anterior and posterior rounded ends similar in breadth ; but the 

 inner medial side is rather flatter than the outer one. 



Plagiaulax Becklesii (?). Plate IV, figs. 15, 15 a. 



To this species I refer, with doubt, a specimen of part of the crown of the incisor, a small 

 part of the anterior premolar {p 2), the penultimate (jy 3) and major part of the last pre- 

 molar (/? 4) apparently of the left side, imbedded in their natural relative positions in a 

 portion of the Purbeck shale, with the outer surface exposed (PI. IV, fig. 15, nat. size. 

 15 a, raagn. 3 diam.). 



