38 Mr. A. S. Woodward on the 



from its removal {e. g. no. 25658) ; and the circumstance has 

 already been briefly noted by Le Hon *. Agassiz f, how- 

 ever, founded the species M. pimctafus upon such an abraded 

 fragment probably referable to the upper jaw of 31. stnatus, 

 and Delfortrie | and Issel § have more recently described 

 fossils that appear to be similarly imperfect under the names 

 of M. microrhiziis and M. granulosus. 



A less amount of abrasion of the grinding-surface often 

 imparts to it a remarkable smoothness, which has also been 

 occasionally relied upon as a specific character in determining- 

 dental plates. All the types of M. Dixoni^ for example, are 

 remarkably smooth ; and though this feature was not espe- 

 cially alluded to in the original diagnoses of Agassiz and 

 Dixon, the circumstance seems to have been sometimes 

 regarded as an essential peculiarity of the species ; fossils 

 truly belonging to this form, but having the grinding-surface 

 preserved, and thus showing striations, have been wrongly 

 referred to M. striatus, as is proved by specimens in the 

 National Collection. The specific name of stnatus in fact 

 might have been as appropriately applied to M. Dixoni and 

 others as to the form that now bears it, if only unworn speci- 

 mens had originally been available ; though it so happens 

 that other peculiarities in the species thus named render it 

 nevertheless valid. 



A further effect of post-mortem wear, or even perhaps of 

 masticatory trituration, has led to the founding of still another 

 species by Agassiz — the so-called M. suturalis ||. This is 

 described as possessing teeth in all respects similar to those of 

 M. toliapicus, but united by jagged sutures rather than 

 straight edges. Such a peculiarity is to be observed more or 

 less in all the specific types when the dentition is deeply 

 worn, as already hinted by Issel and Le Hon, and the frag- 

 ment just referred to may undoubtedly be placed in the well- 

 known species from Sheppey. Leidy's M. ser7'atus^ may 

 also be mentioned in this connexion, the chief character in 

 the diagnosis being similarly misleading. 



Among individual variations liable to be quoted as of 



* H. le Hon, ' Pr6liminaires d'un Memoire sur les Poissons Tertiaires 

 de Belgique,' 1871, p. 13. 



t L. Agassiz, Eech. Poiss. Foss. vol. iii. p. 3'22, pi. xlvii. figs. 11, 12. 



X E. Delfortrie, " Les Broyeurs du Tertiaire Aquitanieu,'' Actes Soc. 

 Linn. Bordeaux, vol. xxviii, (1871), p. 225, pi. x. iig. 37. 



§ A. Issel, "Cenui sui Mi/iiobates fossili dei terreni terziarii Italiani," 

 Ann. Mus. Civ, Stor. Nat. Geneva, vol. x. (1877), p. 335. 



II L. Agassiz, torn. cit. p. 322, pi. xlvi. tigs. 12-10. 



^ J. Leidy, Journ. Acad. Nat, Sci. Philad. vol. viii. (1877), p. 239, 

 pi. xxxii. fig. 5. 



