266 ME. G. E. DI3LEI ON THE TEETH OE PTYCHODUS [May I9IT, 



Pttchodus dectteeens, var. depeessus (Dixon). (PI. XYTI, figs. 1 a 

 & lb.) 



Though described as belonging to a distinct species, Pt. depressiis, 

 by Dixon, 1 the teeth of this form only differ from those typical of 

 Pt. decurrens in being somewhat more flattened and depressed. They 

 are common in the zone of Holaster subglobosus, and I have found 

 interesting associated sets in the pit at Bluebell Hill, Burhara 

 (B.M. P 10255), and in Peter's Pit, Wouldham (B.M. P 10336). 

 The latter specimen comprises fifty teeth, of which thirty-two 

 occur still in their natural arrangement, showing a small portion 

 of the dentition of the upper jaw (PI. XYII, fig. 1 a). From front 

 to back this dentition is very convex, and the teeth do not appre- 

 ciably increase in size backwards. The usual median row of small 

 teeth is partly obscured by the crushing together of the large teeth 

 of the inner paired series. 



Pttchodus deciteeejss, var. oweki (Dixon). (PI. XVIII, figs. 1-11 

 & PI. XIX, fig. 8.) 



The teeth described as Pt. oiveni by Dixon 2 have not hitherto 

 been found in their natural arrangement ; but I have selected the 

 isolated specimens shown in PI. XYIII, figs. 1-11, to prove that 

 there is every gradation between them and the normal teeth of 

 Pt. decurrens. Indeed, it seems probable that the peculiarly 

 irregular and broken ridges that characterize Pt. oweni occur chiefly 

 on the principal median teeth ; for in one set, which evidently 

 belongs to a single individual (B.M. 39137-38), many of the 

 larger teeth correspond exactly with those described as Pt. oweni, 

 while several of the smaller lateral teeth are typically those of 

 Pt. decurrens. I have also observed the same variability of the 

 ridging on the teeth in other associated sets. 



The type tooth and three other specimens in the Bowerbank 

 Collection are labelled as having been obtained from Snodland, 3 

 Rochester. I have found two similar teeth from this locality, where 

 only the Lower Chalk was worked at the time. 



From Oxted I obtained, quite recently, an associated set of 

 fourteen teeth of Pt. decurrens which proved to be worthy of note. 

 Three of the central teeth of the lower dentition are of an extra- 

 ordinary size, the largest being 51 mm. in length and 42 mm. in 

 breadth (PL XIX, figs. 1-3). The grooving on the posterior half 

 is of the typical form, while the next four or five adjoining rows are 

 sharply curved towards the anterior portion, their ends being sharply 

 recurved and extending to the front edge of the tooth. Nearly 

 the whole of the anterior half is occupied by numerous secondary 

 grooves radiating from the centre, and thus partakes of the features 

 belonging to the variety oweni. Of the remaining teeth, there are 



1 ' Geology of Sussex' 1850, p. 363 & pi. xxxi, fig. 9. 



2 Ibid. p. 364 & pi. xxxi, fig. 2. 



8 Hailing is the proper locality, Snodland being situated on the Gault. 



