Vol. 67.] THE TEETH OF PTYCHODUS. 263 



vS. On the Teeth of PTYCHODUS and their Distribution in the 

 English Chalk. By Geobge Edwabd Dibley, E.G.S. (Head 

 March 8th, 1911.) 



[Plates XVII-XXIL] 



Introductory Remarks. 



Among the remains of fishes found in the Chalk, the teeth of 

 Ptijchodus are so conspicuous and so easily recognized by the 

 quarrymen that they haye long been collected in large numbers 

 and distributed to various museums. Most of these fossils, however, 

 bear no record of the exact locality or zone from which they were 

 obtained, while groups of associated teeth have often been scattered 

 without any note of the circumstances of their discovery. I have, 

 therefore, devoted much attention during the past twenty years to 

 the careful collecting of teeth of Ptijchodus from the chalk-pits of 

 the South-East of England, and I now propose to discuss their 

 classification and the zonal range of the various species. 



I have collected especially in the Gravesend, Rochester, and 

 Medway -Valley area, where there are great excavations in a 

 continuous series of Chalk-deposits, from the base of the zone of 

 Ammonites rhotomagensis to the top of that of Micraster cor- 

 anguinum. I have also obtained specimens from the Caterham 

 Valley, Oxted, Merstham, and Betchworth ; from Berkshire, Hamp- 

 shire, Bedfordshire, and Cambridgeshire ; and I have studied all 

 the material in the British Museum, the Museum of Practical 

 Geology (Jermyn Street), and the museums of Salisbury, Brighton, 

 and Rochester. I have been particularly fortunate in discovering 

 fifty sets of associated teeth, and have examined many more in 

 the museums just enumerated. 



The arrangement of the teeth of Ptychodiis in the mouth is 

 already known from specimens of Pt. decurrens found in the English 

 Chalk, 1 and from others of Pt. mortoni found in the Chalk of Kansas 

 (U.S.A.). 2 I have obtained confirmatory evidence of it in a naturally 

 arranged group of teeth of Pt. decurrens, from the Lower Chalk of 

 Wouldham near Rochester; and in another group of the same 

 species given to me by Mr. G. Bishop, Manager of the Lime- Works 

 at Merstham (Surrey). 



It appears that the teeth are arranged in antero-posteriorly 

 directed parallel rows, in symmetrical pairs on each side of a median 

 row, which contains the largest teeth in one jaw (presumably the 

 lower) ; while in the other (presumably the upper) the median row 

 consists of diminutive teeth somewhat long and narrow, with the 

 grooving of the enamel confined to the centre and across the width. 



1 A. Smith Woodward, Q.J.G. S. vol. xliii (1887) pp. 121-30; and ibid. 

 vol. lx (1904) pp. 133-35. 



2 S. W. Williston, Kansas Univ. Geol. Surv. vol. yi (Palceont.) pt. ii (1900) 

 pp. 234-40. 



