PTYCHODUS. 



237 



Another form of lower median tootli, with a narrow granulated marginal area and 

 somewhat more marked gyration of the eight transverse ridges, has been wrongly 

 referred to P. polygyrus (A. S. Woodward, op. cit., 1889, pi. v, fig. 7). Except 

 that it is a little wider in proportion to its length and sometimes has the hinder 

 ridges more bent, this closely resembles the corresponding teeth in part of a lower 

 dentition described by Leriche from the Turonian of France (Text-fig. 7-4-). 

 Similar lower median teeth also occur in an associated series in the Mnsenm of 

 Practical Geology from a Turonian zone at Wantage, Berkshire ; and a larger 

 specimen, with the ends of the seven transverse ridges less curved, is shown in 

 PI. L, fig. 4. Still another form of lower median tooth shown by Agassiz (turn. 



Fig. 75. Ptychodus latissimus, Agassiz ; teeth of tipper jaw, found associated with tl.ose shown in Text-fig. 74, 



about one half nat. size. After M. Leriche. 



cit., pi. xxvf?, figs. 5, G), and again in PI. L, fig. G, is crossed by only six very 

 coarse ridges, of which the foremost or hindmost tends to be broken ; and in this 

 form the marginal area is sometimes as finely granulated as in P. mammillaris (PI. L, 

 fig. 5). The elevation of the middle portion in the latter tootli is unusually sharp. 

 The teeth of the lower paired lateral rows are not satisfactorily known, but 

 their arrangement is doubtless approximately correct in the original of Text-fig. 74, 

 in a specimen described by Dibley {Joe. cit., 1911), and in the associated set from 

 Wantage already mentioned in the Museum of Practical Geology. Some of these 

 teeth also occur in association with the lower median tootli figured in PL L. fig. 1, 

 and two are shown in figs. 2, 3. A tootli which seems to belong to the inner or 

 first paired row (fig. 2) is slightly wider than long, and its total width equals 

 about two thirds that of the median tooth. It is crossed by five complete ridges,. 



