Fossil Teeth 0/ M jliobatis. 39 



specific value the most striking perhaps is the variability 

 sometimes so conspicuous in the antero-posterior measurements 

 of the series of median teeth. This is a feature occasion- 

 ally exhibited in every form, and there are good illustrations 

 among the national fossils in dental plates of M. Dixoni and 

 M. toliapiciis^ besides another specimen originally figured by 

 Dixon under the name of M. Edwardsii. In the imperfect 

 diagnosis of the latter species, however, the peculiarity is 

 mentioned as one of the leading distinctive points *. 



Equally inconstant are the small differences in the antero- 

 posterior curvature of the median teeth, which are sometimes 

 referred to with undue emphasis. In some species it is true 

 tliere is a greater tendency towards the curvature of the plates 

 than in others^ and the present materials are insufficient to 

 decide whether or not the sharp flexure of the extremities of 

 the median teeth in certain forms is likewise a more or less 

 fixed character ; but it appears to be unsafe to rely upon the 

 point when the specimens for study are few and fragmentary. 



A prominent feature that seems to be entirely due to the 

 effects of " overgrowth " has also been cited as the main 

 characteristic of one other fossil form — the M. irregularis of 

 Dixon f. In this species the median plates are slightly more 

 than nine times as broad as long and very irregular both in 

 their borders and surface-contour. But, as will be shown in 

 the sequel, almost every gradation can be found between the 

 type specimen, which forms an extreme, and the more normal 

 teeth known as J\L striatus \ and there can thus be little 

 hesitation in regarding this unique form as a very large 

 variety of the latter — perhaps an unusually aged individual. 

 The same irregularity, indeed, appears to exist in the teeth of 

 overgrown examples of other species, e. g. M. toliapicus ; for 

 a single specimen probably referable to the last-named form, 

 in which the ordinary adult ratio of length to breadth in the 

 median series is about 1 : 6, exhibits a corresponding ratio of 

 1 : 7*5, and has all the inequalities of surface-contour pre- 

 sented by Dixon's fine fossil. 



But the most fundamental consideration of all to be taken 

 into account when determining the fossil dental plates of 

 Myliohatis relates to their mode of growth ; and this I have 

 not found mentioned in any contribution to tlie palaeontology 

 of the genus, except that of Issel quoted above j. As 



* F. Dixon, Fobs. Suss. p. 199. 

 t F. Dixon, op. cit. p. 199, pi. xi. fig. 15. 

 • i A. Irisel, Ami. Mus, Geneva, vol. x. (1877), p. 316. 



