242 FOSSIL FISHES OF THE ENGLISH CHALK. 



Others, such as the tooth of the upper inner paired row shown in fig. 8, are 

 remarkable for their large size and some irregularity. 



A comparatively depressed and flattened tooth, apparently of the upper inner 

 paired row, was named P. depressux by Dixon (op. clt., 1850). Similar teeth occur 

 in natural order in a portion of the upper dentition discovered by Mr. Dibley 

 (PI. LI, figs. 9-12), and they all exhibit the characteristic markings of P. decurrens, 

 with a very narrow granulated marginal area, except in the small median teeth. 

 The latter (fig. 10), which closely resemble the median tooth shown in Text-fig. 

 70, are partly obscured in the fossil by the squeezing together of the inner paired 

 row (\'). The teeth of this row are slightly broader than long, and a detached 

 example, drawn from several aspects in figs. 11-llc, shows all their characteristic 

 features. Their transverse ridges are from nine to eleven in number, while those 

 in the anterior half of each tooth curve forwards at the ends, which often diverge 

 to admit short intercalated ridges. The teeth of the second paired row (n') are 

 similarly marked, but are much smaller and a little longer in proportion to their 

 breadth. The smaller teeth of the third and fourth rows (in', iv') are also long in 

 proportion to their breadth, but obliquely distorted. The average transverse 

 measurements (in millimetres) of the teeth of the several rows are as follows : o', 8 ; 

 i', 25; ii', 17; m', 13; iv', 11. 



Part of a small lower dentition Avhich seems to be referable to the depressed- 

 toothed variety of P. decurrens is shown of the natural size in Text-fig. 77. The 

 transverse ridges of the crown are fine and numerous, from 13 to 14 in number 

 in the teeth of the middle row, and reaching the extreme margin, frequently with 

 intercalations at the ends, in all the teeth. The average transverse measurements 

 (in millimetres) of the several rows preserved are as follows: — o, 21; i, 14; n, 

 10; in, 9. 



The irregularities observable in the arrangement of the transverse ridges of 

 the teeth just described are still more conspicuous in some other teeth of the 

 depressed variety. Five such specimens from an associated set are shown in 

 PL LIT, figs. 1-5, and appear to be referable chiefty to the upper jaw. In a tooth 

 of the inner paired row (fig. 2), the anterior ridges begin to be irregularly 

 branched, with two or three intercalated tubercles. In the teeth apparently of 

 the upper second and lower first paired rows (figs. 3, 1) the ridges are more 

 regular, but in those perhaps of the upper third paired row (fig. 4) and outer 

 rows (fig. 5) they are very irregular, again with intercalated tubercles. Vermi- 

 culating ridges and more numerous intercalated tubercles are also seen in 

 the two small associated teeth represented in PL LII, figs. 7, 8 ; and the extreme 

 of this type of irregularity is reached in the larger teeth which have received the 

 name of P. oiveni (Dixon, op. cit., 1850). Dixon's type specimen seems to belong 

 to the upper inner paired row, and two other teeth of the same row, which were 

 probably found with it, are shown in PL LII, figs. 9, 10. Here there is scarcely 



