EEL TRIBE. 445 



a final classification is still a desideratum. By Professor Cope the families have 

 been arranged in a number of sectional groups, mainly distinguished by the structure 

 of the skeleton ; and a modification of this arrangement is adopted here, although 

 fewer groups are recognised. It is, however, impossible to enter here into the 

 consideration of the osteological features by which these sections are distinguished, 

 and we are accordingly compelled to rely mainly on external characters. 



The Eels and their Allies, — Families MURuENlDJE, 

 SymbranchidjE, and Gymnotidas. 



The whole of the members of these three families are characterised by the 

 elongated, " eel-like " form of the body ; but it is quite probable that this external 

 similarity is due to parallelism in development, and that the three families have 

 been independently derived from very different types of more normally formed 

 fishes. The first family, which includes the true eels, mursenas, and congers, is 

 characterised by the normal structure of the upper jaw, which is formed in front 

 by the premaxillse (more or less confluent with the vomer and ethmoid) and 

 laterally by the toothed maxillae. The median fins, when present, are either 

 confluent or separated by the projecting tail ; the pectorals may or may not be 

 developed; but the pelvic pair is invariably wanting. There are no accessory 

 breathing -organs; the stomach has a blind appendage; the vent is generally 

 situated far back, but may be near the pectoral fins ; and the ovaries have no 

 ducts. Externally the skin may be either completely naked, or may contain 

 rudimental scales. In the skeleton the pectoral arch is unconnected with the 

 skull, and attached to one of the earlier vertebrae. Eels are found in the fresh 

 waters and seas of the greater part of the temperate and tropical regions ; some 

 living at abyssal depths in the ocean. The young of some forms are pelagic for 

 a portion of their existence ; and it is believed that a large number of the so- 

 called Leptocephali (see p. 322), or glass-eels, are abnormal larva? of this family. 

 Geologically the family is a comparatively ancient one, true eels having been 

 discovered in the Chalk of the Lebanon, as well as in the Tertiaries of Europe. 

 Congers referred to the existing genus Ophichthys have been described from the 

 middle Eocene of Monte Bolca, and there is also an extinct genus from the latter 

 deposits, and a second from the London Clay. 



The mursenas are large marine eels, remarkable for their bright 



Mursenas. . . . 



spotted or mottled coloration, and taking their name from the species 



here figured (Murcena helena), which was so called by the ancient Romans. 



Belonging to a small section of the family characterised by the gill-openings into 



the pharynx being in the form of narrow slits, they are specially distinguished by 



the median fins being well developed, and the total absence of pectorals. The 



skin is scaleless ; the mouth is well furnished with teeth ; and there are two 



nostrils on each side of the muzzle, the front pair being tubular, while the hinder 



ones may be either tube-like or mere flat openings. The mursenas, of which there 



are more than eighty species, are distributed over all tropical and temperate seas, 



and a few ascend tidal rivers. The majority of them are armed with formidable 



teeth — which frequently alter considerably with age — adapted for seizing the fish 



