832 MESSRS. HANCOCK AND ATTHEY 



Six or seven other specimens of these secondary teeth have 

 occurred scattered in the same shale in which the primary teeth 

 are found. The secondary teeth have a certain resemblance 

 generically to the primary teeth, and specifically they have cha- 

 racters in common with their respective primary teeth. Never- 

 theless they are scarcely generically distinguishable from the 

 Petalodus of authors, though they are, as already stated, oblique. 



Having said thus much with respect to the external characters 

 of the teeth themselves in the two genera in question, we must 

 now make some remarks about their arrangement in the mouth. 

 In Janassa it is clearly demonstrated, both by the specimens and 

 figures before alluded to, that the teeth are similarly arranged 

 in both the upper and under jaws. In this genus they are 

 placed in slightly arched transverse rows, the largest symmetri- 

 cal primary tooth being situated on the median antero-posterior 

 line, and projecting a little in advance of the others. On each 

 side of this there are two similar teeth, but somewhat less, the 

 outside one being twisted obliquely ; the row is then terminated 

 on either side by one of the Petalodontoid form. There are 

 therefore seven teeth in each row, including both kinds— five 

 primary, two secondary. Miinster represents five or six such 

 rows in close succession from back to front, the teeth and rows 

 gradually diminishing in size forward. It is evident, then, that 

 the arrangement of the buccal armature more closely resembles 

 that of the Rays than the Cestracionts or the Sharks ; and indeed 

 notwithstanding the difference in the teeth themselves, in their 

 arrangement they agree in a remarkable manner with those in 

 Myliohatis aquila and Zygohatis marginata — a relationship which 

 was recognized by Agassiz.* In the extraordinary dental ap- 

 paratus of these two interesting forms the teeth or plates are 

 placed crosswise on the anterior portion of the jaws in rows suc- 

 ceeding each other from back to front. The largest primary 

 tooth is median : on each side of it there are two other primary 

 teeth, both of which are small in the first genus, and only one 

 in the second ; all these teeth are characterized by having six 

 sides ; and each row is flanked by a small or secondary tooth, 



* Poissons Fossils, Tome III., p. 375. 



