Fossil I'eeth o/' Myliobatis. 41 



Alyliohatis Dixoni^ Agassiz. (PI. I. tigs. 1-4.) 



18-33-43. Myliohatis Dixoni, Agassiz, Poiss. Foss. vol. iii. p. 319. 

 1833-43. Myliohatis Jieteropleurus, Agassiz, torn. cit. p. 323, pi. xlvii. 



figs. 6-8. 

 1850. Myliohatis Dixoni, Dixon, Fosa. Suss. p. 198, pi. x. figs. 1, 2, 



pi. xi. fig. 14, pi. xii. fig. 3. 

 1850. Myliohatis contractus, Dixon, op. cit. p. 200, pi. xi. fig. 17. 

 1850, Myliohatis striatus, Dixon (non Agass.), q/j. cit. pi. xii. fig. 2. 



This species was named by Agassiz on inspecting the 

 drawings of Mr. Dixon's fossils prepared for the well-known 

 * Geology and Fossils of Sussex,' all of which represented 

 the upper jaw. Its main peculiarities were rightly noted as 

 (i.) the strongly arched [upper] coronal surface, and (ii.) the 

 great length of the median teeth compared with their breadth 

 — the ratio rarely or never being more than 1 : 5. There are 

 three rows of lateral dental plates on each side, which are all 

 much elongated in shape. 



The national specimens now render it possible to advance 

 a step further, by indicating the changes in the relative 

 dimensions of the median teeth resulting from differences of 

 age (or absolute size), and the following series of measure- 

 ments will show eight successive stages. In this table, as in 

 each of the others, the numbers are expressed as decimal 

 fractions of the metre, and the length in every case is approxi- 

 mately the average of three or more plates. 



U^per Dentition of Myliobatis Dixoni. 



The lower dentition has not hitherto been recognized as 

 such, although two or three examples have been figured under 

 other specific names. There can be little doubt, however, 

 that the following series of specimens is rightly so deter- 

 mined, and the measurements, it will be noticed, correspond 

 very closely with those of the upper dental plates. Four of 

 these specimens (nos. L, V., VII., and VIll.) are shown of 

 the natural size in PI. 1. figs. 1-4, and a transverse section of 



