40 
Cambridge ; but neither this, nor any similar known fossil, 
reveals the precise form and proportions of the several dental 
series in the species in question. 
II. Ptychodus Polygyrus, Agassiz. (Figs. 15—20). 
The second group of teeth under consideration comprises 
nearly seventy examples, and was discovered in the debris from 
a fall in the chalk cliffs between Folkestone and Dover. It 
represents the form of P. poly gyrus, to which Agassiz applied 
the varietal name of marginalis ; and six of the principal teeth 
are shown in figs. 15—20. No member of the upper median 
series appears to be preserved; but there are several specimens 
of the lower median tooth (fig. 15), and these are of interest as 
being identical with the large broad tooth, which was erroneously 
assigned to P. latisshnus by Agassiz,* and has recently been 
removed to P. polygyrus by the present writer, from a study of 
specimens in the British Museum.f The originals of figs. 16 
and 17 are two forms of teeth probably referable to the first 
lateral series of the upper j aw ; and the second specimen exhibits 
two unsymmetrically placed areas of wear. Figs. 18 and 19 
represent two somewhat smaller teeth that may have occupied 
a similar position in the lower jaw ; and a tooth evidently of 
the second lateral series, either upper or lower, is shown in fig. 
20. The latter is narrower in proportion to its length than the 
principal teeth already noticed; but the characters of the 
crown and its superficial ornamentation are obviously the same. 
Several of the smaller lateral teeth, more outwardly placed, 
agree precisely with that just referred to in form and coronal 
configuration ; and it is only in the outermost smallest teeth 
that the characteristic features of the dental crown in the species 
are unrecognisable. 
In all these teeth, it will be observed, the crown is little 
elevated, with a broad fiat top; and the transverse coronal 
ridges vary from about eight to ten in number, while the 
peripheral area is coarsely granulated and rugose, without any 
diverging furrows. Although, as remarked by Agassiz, P. 
polygyrus is a specific name applied to many closely related 
* Poiss. Fossiles, vol. iii. (1843), p. 167, PI. xxv. a, fig. 8. 
t Catalogue Fossil Fishes British Museum, pt. i, (1889), p. 143. 
