Vol. 67.] AND THEIE DISTRIBUTION IN THE ENGLISH CHALK. 267 



seven large teeth belonging to the lateral rows, of which six are 

 much deformed as regards shape ; two are shown in PI. XIX, 

 figs. 4 & 5. The grooving also is very abnormal in many of the 

 teeth. 



There is a set in the British Museum (Bowerbank Collection, 

 39138) from Wouldham, and another which is labelled as coming 

 from Hailing, though it evidently belongs to the previous set : as can 

 be seen from the matrix, the peculiar staining, and the abnormality 

 in shape and grooving resembling the Oxted set, but with an addi- 

 tional feature in the form of small papiilne developed in the grooves 

 (PI. XVIII, fig. 4 & PI. XIX, fig. 15). 



Ptychodus decurrens, var. multistriatus (A. S. Woodward). 

 (PI. XVIII, figs. 12 & 13; PI. XIX, figs. 6, 10, 12, & 14.) 



The so-called Pt. multistriatus (A. S. W.) x from the zone of 

 Holaster subglobosus comprises large teeth marked with numerous 

 fine ridges, which are sharply recurved into a concentric arrange- 

 ment. I am now able to prove, however, that these are merely 

 from the inner rows of a dentition in which the outer teeth conform 

 to the typical Pt. decurrens plan : for, in 1905, I obtained from the 

 same zone at Holborough, near Rochester, an associated set of 

 seventeen teeth, of which only those of the large median lower row 

 are typical examples of the so-called Pt. multistriatus ; while those 

 of at least six paired rows exhibit every gradation between this 

 condition and that of the true decurrens- ridging, which characterizes 

 the outermost rows. Some of these gradations may even be seen 

 in the type-specimen in the British Museum (P 2681). 



Ptychodus decurrens, var. ljevis (A. S. Woodward). 



The very small and remarkably smooth teeth from the Lower 

 Chalk, described as Pt. Icevis (A. S. Woodward) 2 , exhibit the 

 characteristic grooving of Pt. decurrens, and can only be regarded as 

 a dwarfish variety. I have observed a similar abnormal smoothness 

 in a tooth of Pt. mammillaris in my own collection. Several 

 examples are now known ; and I have obtained them from the 

 zone of Holaster subglobosus, both at Betchworth and at Burham. 



Geological Range. 



Ptychodus decurrens occurs chiefly in the zone of Holaster sub- 

 globosus. It is also found, but rarely, in the Actinocamax-plenus 

 Marls, and still more rarely in the base of the zone of Ithy nchonella 

 cuvieri. It has not hitherto been discovered at any higher horizon. 

 Two teeth of this species are in the Museum of Practical Geology 

 (Jermyn Street), aud are said to have been obtained from the 



1 Catal. Fossil Fishes Brit. Mus. pt. i (1839) p. 146 & pi. v, figs. 4-6. 



2 Proc. Geol. Assoc, vol. xiii (1893-95) p. 192 & pi. v, figs. 5-tf. 



