Fossil Teeth of Myliobatis. 37 



ally, moreover, the most distinguished naturalists have been 

 led by imperfect specimens to enumerate as specific characters 

 features that are wholly due to the effects of post-mortem 

 abrasion ; and other equally unreliable points have likewise 

 been emphasized, owing to misapprehensions as to their 

 significance and constancy. 



Such mistakes are quite inevitable whenever materials are 

 scarce and fragmentary, and especially when the observer has 

 had but few recent specimens for comparative study. When, 

 however, it is possible to examine and compare a large series 

 of fossils from the same formation and locality, data are pro- 

 vided for much more certain and philosophical conclusions. 

 And as the National Collection now comprises a very large 

 number of the dental plates of Myliohatis from the London 

 Clay of Sheppey and the Middle and Upper Eocenes of 

 Bracklesham and Barton, the present seems a favourable 

 opportunity for attempting some slight revision of the group. 

 H ere are preserved the type specimens of nearly all the species 

 hitherto described from these formations, besides many other 

 beautiful fossils from the cabinets of Mr. Frederic Dixon, Dr. 

 Bowerbank, Sir Philip Egerton, and the late Earl of Ennis- 

 killen ; and the whole are suggestive of some interesting con^ 

 siderations, which do not appear as yet to have been sufficiently 

 recognized, at least by those who have examined the genus 

 from a palasontological point of view. I therefore propose, in 

 the present communication, to offer a few remarks upon the 

 subject, based upon a careful study of the fossils I have 

 recently been able to make; and the conclusions will lead 

 to some slight reduction in the number of specific types 

 supposed by previous authors to be represented. 



Specific Characters afforded hy Dentition. 



Referring in the first place to the more general questions, 

 it will be convenient to commence with a notice of the decep- 

 tive appearances due to post-mortem abrasien. To this may 

 be added some remarks upon individual variations and differ- 

 ences in the teeth corresponding to differences of age. And 

 the discussion will appear to result in determining at least 

 three points of certain taxonomic value. 



One of the most conspicuous of the accidental characters 

 is the granular ox punctate ornamentation produced by the 

 removal of the superficial gano-dentine and the exposure of 

 the vertical medullary tubes. There are several specimens in 

 the British Museum demonstrating the process of wear and 

 showing portions both of the original surface and that resulting 



