40 Mr. A. S. Woodward on the 



already pointed out by Dr. Giiiither * there is no median 

 series of larger teeth in very young individuals, all the plates 

 being originally of nearly equal size and more or less regu- 

 larly hexangular. But as growth proceeds the middle row 

 begins to exhibit the familiar lateral elongation ; and with the 

 progressive increase in the size of the animal this peculiar 

 character becomes more and more marked, until in the largest 

 individuals — generally the most aged — there is the maximum 

 ratio between length and breadth. In any one species, 

 therefore, the median teeth have different relative dimensions 

 at different ages, and in dealing with the fossils it is obviously 

 necessary to take note of absolute size when stating these 

 relative measurements for diagnostic purposes. 



Another peculiarity which appears to be of equal import- 

 ance to the last and can be employed with less restrictions is 

 the form of the small dental plates arranged in the lateral 

 rows. Species with broad lateral teeth seem to retain them 

 correspondingly broad throughout life, and those in which 

 they are narrow or small have them narrow and small at all 

 ages ; and when there is any prominent obliquity or irregu- 

 larity this likewise exhibits but the slightest change as the 

 successive plates follow one another during growth. 



Lastly, the coronal surface- contour of the lower teeth may 

 often be relied upon to a certain extent as a diagnostic 

 character w'hen the specimen is unabraded. The flatness or 

 transversely arched form of the crown and its relative thick- 

 ness are features of some specific value, though even in this 

 respect I have found considerable variations in the young 

 stages of one of the English species [M. striatus) described 

 below, 



Itevision of English Eocene Species, 



Applying the foregoing results to the revision of the English 

 Eocene fossils it appears that as yet only four distinct species 

 can be recognized with certaintj^, while three others remain 

 incompletely defined and cannot at present be accepted. The 

 well-marked types are M. Dixonij M. striatus^ M. toliapicus^ 

 and a hitherto undiscovered form which I propose to name 

 M. latidens ; and it will be convenient to treat of them in the 

 order mentioned. 



* A. Giinther ' Catalogue of Fishes iu the British Museum,' vol. viii. 

 (Ib70), p. 489. 



