GENUS TRYGON. 175 



and size, found in the rivers of Guiana, but none 

 have now come under notice on this expedition but 

 the specimen from which the drawing was made, 

 which is of the natural size ; it differs in colour from 

 all the others." 



Humboldt discovered another small species of 

 Gymnotus in the river Madelina, which he has 

 named G. cequilaliatus^ to distinguish it from the 

 others, which have a considerable inequality in ihe 

 length of the lips, or rather jaws. It is used as 

 food, and in some parts it was considered as a luxury. 

 It possessed no galvanic properties. The form is 

 somewhat like that which we have now figured, 

 but at the caudal extremity tapers nearly to a 

 narrow thread, from which it has received the pro- 

 vincial appellation of " El Raton." The colour 

 above is dark olive, shading into silvery below the 

 lateral line. 



We have now examined all the drawings belong- 

 ing to one great natural division of fishes, those with 

 osseous skeletons ; in the fresh waters of any country 

 the cartilaginous species are of rare occurrence, their 

 large size and predaceous habits requiring for them 

 a wider range ; several species of small rays, however, 

 are found at a considerable distance inland in several 

 of the Guiana rivers ; three, those of which we pos- 

 sess drawings, approach nearest to the genus Trygvn, 

 while a fourth we can reconcile with no characters 

 to wm>\h we have access; and it is possible that 



