Vol. 60." JAWS OF PTYCHODUS FROM THE CHALK. 133 



11. Oa the Jaws of Ptychodus from the Chalk. By Arthcr 

 Smith Woodward, LL.D., F.R.S., F.L.S., F.G.S.. of the British 

 Museum (Natural History). (Read January 20th, 1904.) 



[Plate XV.] 



In 1^ v 7 I pointed out that the teeth of Ptychodus from the Chalk 

 were arranged in the mouth not like those of the Cestraciont sharks, 

 but rather like those of some of the Myliobatid rays. This arrange- 

 ment has subsequently been observed in new specimens, both in 

 England and America." Hitherto, however, no traces of the carti- 

 laginous jaws have been found in association with the dentition, 

 and their shape and relations have thus remained unknown. Quite 

 lately, a new specimen, partly showing the jaws, has been obtained 

 from the Lower Chalk of Glynde by Mr. Henry Willett, and he 

 has kindly submitted it to me for study, to supplement my former 

 description, which was chiefly based on the Willett Collection in the 

 Brighton Museum. 



The new fossil evidentlv belongs to a small variety or voung indi- 

 vidual of Ptychodus deeurrens, the species previously discussed, and 

 its principal characters are shown in the accompanying plate (XV). 

 It comprises fragmentary remains of both jaws, each bearing many of 

 the characteristic teeth arranged in their natural order. The rami of 

 the jaw which may be identified as mandible (PI. XV, figs. 1 &: 2. md > 

 are remarkably slender, and meet in an acute angle at the symphysis, 

 which is shown to have been elongated, though it is disintegrated 

 by the formation and oxidation of iron-pyrites. The upper jaw or 

 pterygo-quadrate cartilage (fig. 2. ptq ) is represented only by shape- 

 less fragments. The dentition i> confined exclusively to the sym- 

 physial region, where the teeth are arranged in the usual parallel 

 antero-posterior rows. 



( )f the lower teeth, some are preserved in natural order above 

 the hinder part of the symphysis, while a few arc scattered in front 

 on the decomposed anterior end of the jaw. Of the large median 

 series (fig. 2, o), three teeth are exhibited, displaying all the 

 typical characters of P. decurrens. To the left of these are teeth of 

 four paired lateral series (i-iv) in natural arrangement; while a 

 very small displaced tooth (v) seems to represent a fifth series at 

 the extreme outer border. It is to be observed that the dentition 

 does not curve backward at the side to spread along the mandibular 

 ramus, although the more laterally-placed teeth exhibit the usual 

 slight oblique distortion. If the larger scattered teeth on the sym- 

 physis belong to lateral series i, as seems probable, it is also to be 



1 'On the Dentition & Affinities of the Selachian Genus Ptychodus, Agaas 

 Quart. Journ. Geol. Soc. vol. xliii 1887 pp. 121-30 £ pi. x. 



- S. W.Williston, Cretaceous Selachians & Pycnodonts ' Univ. Geol. Surv. 

 Kansas, vol. vi (1900) p. 230 & pis. xxy-xxtu. 



