164 



ORDERS OF FISHES. 



arched transverse rows in both upper and lower jaws. The 

 upper surface of each tooth is hollowed out in front, with a 

 sharp anterior edge, and is crossed behind by transverse ridges. 



Fig. 531. — A few of the central teeth of 

 Janassa linguieformis, about the natural 

 size. Carboniferous. (After Hancock and 

 Atthey.) 



Fig. 532.— Teeth of a fossil Ray {Mylio- 

 batis Edwardsii). Eocene. 



Janassa seems, upon the whole, to be most closely allied to 

 Myliobatis, though its external teeth approximate in form to 

 those of the " Petalodonts." 



In the Jurassic rocks occur the remains of Eays, which 

 have been referred to the genera Sriualoraia, Spathobatis, 

 Arthroptcrus, &c. In the Tertiary rocks the remains of Eays 

 are tolerably abundant, and consist almost exclusively of the 

 dental plates. These consist (fig. 532) of generally flat plates, 

 usually somewhat rhomboidal in shape, often placed close 

 together and sometimes united laterally by sutures, so as to 

 " form a kind of mosaic pavement on both the upper and 

 lower jaws " (Owen). Most of the fossil forms belong to the 

 genus Myliobatis, which comprises the living Eagle-rays. All 

 the fossil species of this family belong to the Tertiary period. 



Order IV. Dipnoi (= Protopteri, Owen). — This order is 

 a very small one, and includes, among recent forms, only 

 the singular Mud-fishes (Lepidosiren and Ceratochis) ; but 

 it is nevertheless of great importance as exhibiting a dis- 

 tinct transition between the Fishes and the Amphibia. So 

 many, in fact, and so striking, are the points of resemblance 

 between the two, that until recently the Lepidosiren (fig. 

 533) was always made to constitute the lowest class of 

 the Amjjhihia. The highest authorities, however, now 

 concur in placing it amongst the Fishes, of which it con- 

 stitutes, with its allies, the highest order. The order Dipnoi 



