234 FOSSIL FISHES OF THE ENGLISH CHALK. 



truncated than the outer margin. The large lower median teeth (fig. 2, o) corre- 

 spond exactly with a tooth erroneously referred to P. latissimus by Agassiz (loc. cit., 

 pi. xxv a, fig. 8). Their median portion is gently raised (more elevated than in 

 any of the other teeth), and crossed by eleven or twelve transverse ridges, which 

 are often irregular, sometimes with intercalated tubercles, and always con- 

 spicuously gyrate at the ends. The marginal area is narrow, especially behind, 

 and coarsely granulated. The teeth ascribed to the lower inner paired row (l) 

 are rather larger than those of the upper second row (n') and scarcely more 

 sharply truncated at the internal than at the external margin. They are crossed 

 by eight or nine transverse ridges, which tend to converge in the usual gyrations 

 at the lateral borders, and leave very little space for marginal granulations except 

 anteriorly. The teeth provisionally referred to the lower second paired row (n) 

 are intermediate in size between those already described as belonging to the upper 

 second and third paired rows, and are closely similar to the latter in shape. 

 They bear seven or eight transverse ridges. The small lateral teeth of both jaws 

 (fig. 3) are much wider than long, somewhat obliquely distorted, with from five to 

 seven transverse ridges, which are usually regular and tend to converge at the 

 ends. In most of these teeth the granulated marginal area is conspicuous. 



Some of the teeth of the typical form of P. polygyms are longer in proportion 

 to their width than those just described, with one or two additional transverse 

 ridges ; and a few are less flattened. Through these every gradation appears to 

 be traceable to the variety marginal ix, which is smaller, and is sometimes not 

 readily distinguishable from P. mammillaris. The type specimens of this 

 variety are not accurately figured by Agassiz (lor. cit., pi. xxv, figs. 6 — 8), and one 

 of them, still in the Mantell Collection, is re-drawn in PI. XLYI1I, figs. 12, 12 a. 

 This tooth evidently belongs to the first inner paired row, either upper or lower, 

 and is peculiar in having the wide, flattened median area sharply raised above the 

 marginal area. The marginal granulations are not coarse beads without order as 

 shown by Agassiz; but those interno-laterally are arranged in concentric lines 

 continuing the convergent transverse ridffes, while those at the extreme border 

 are much finer though also elongated in generally concentric lines. 



The finest known associated set of teeth of the variety marginalis was 

 discovered by Mr. G. E. Dibley in a Turonian zone at Wouldham, Kent, and a 

 selection is shown, artificially arranged, but probably in natural order, in PI. 

 XLVIII, figs. 13, 14. Here the transverse ridges exhibit scarcely any irregu- 

 larities, and their terminal gyration is well marked in the principal teeth. In the 

 small upper median teeth (fig. 13, o') the middle area is relatively larger and the 

 anterior border more rounded than in the typical P. poly gyrus. In the large 

 upper teeth of the inner paired row (i') the median portion is gently raised, 

 though flattened ; and it is crossed by eleven regular transverse ridges, of which 

 the gyrate ends often pass into lines of granules on the marginal area. The teeth 



