342 ORDERS OF FISHES. 



the former, the flat surfaces of the body are truly the dorsal 

 and ventral surfaces. In the latter, as before remarked, the 

 body is flattened, not from above downwards, but from side 

 to side, and the head is so twisted that both eyes are brought 

 to one side of the body. 



The tail in the Rays is long and slender, usually armed with 

 spines, and generally with two or three fins (the homologues 

 of the dorsal fins). The mouth is paved with flat teeth of a 

 more or less rhomboidal shape. The integument is often 

 furnished with placoid structures of a peculiar shape, consist- 

 ing of oval or rounded osseous discs, from the centre of each 

 of which springs a curved spine of dentine. The tail also is 

 sometimes armed with a doubly-serrated defensive spine. 



The Rays are known in the fossil condition by their flat 

 crushing teeth mainly, but also by their fin-rays, bony discs, 

 and defensive spines. The earliest trace of the Rays is found 

 in the Carboniferous Rocks, where occurs the doubly-serrated 

 spine which is referred to the genus Plairacanthus (fig. 296, i). 

 In this singular form, however, the spine seems to have been 

 inserted at the back of the head, instead of in the tail, as in the 

 living Sting-rays. In the Jurassic Rocks occur the remains of 

 Rays, which have been referred to the genera Squaloraia, 

 Spathobatis, Arthropterus, &c. In the Tertiary Rocks the re- 

 mains of Rays are tolerably 

 abundant, and consist almost 

 exclusively of the dental plates. 

 These consist (fig. 305) of gene- 

 rally flat plates, usually some- 

 what rhomboidal in shape, 

 often placed close together and 

 sometimes united laterally by 

 sutures, so as to " form a kind 



Fig. 305. Teeth of a fossil Ray (Mylio- / > ,-, 



Ltis Edivardsif). Eocene. of mosaic pavement on both 



the upper and lower jaws " 



(Owen). Most of the fossil forms belong to the genus Mylio- 

 batis, 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 only the singular Mud-fishes 

 (Lepidosiren and Ceratodus) ; but it is nevertheless of great im- 

 portance as exhibiting a distinct 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. 306) was always made to constitute the lowest 

 class of the Amphibia. The highest authorities, however, now 



