XXII APODES 601 
Nearly 150 recent species are known, of which some 50 are 
deep-sea forms, occurring down to 2500 fathoms. Scanty fossil 
remains, referable to recent genera or scarcely different from them, 
are known from the Kocene of Europe. The Cretaceous genus 
Urenchelys, from England and the Lebanon, is interesting as 
representing a more generalised type, the hindmost vertebrae bear- 
ing a pair of expanded hypural bones, showing the diphycercal 
Eels to haye been derived from Fishes with a normal caudal fin. 
The genera are numerous. The following are the principal :— 
Anguilla, Simenchelys, Llyophis, Conger, Coloconger, Congromuraena, 
Uroconger, Heteroconger, Muraenesox, Nettastoma, Nettophichthys, 
Saurenchelys, Nettenchelys, Myrus, Myrophis,\Derichthys, Chilo- 
rhinus, Muraenichthys, Liuranus, Ophichthys, Moringua. 
In the first four genera, small, more or less lineal rudimentary 
scales are embedded in the skin, arranged in small groups, which 
are placed obliquely at right angles to one another, forming a 
curious pattern; but these scales are so small that they escape 
the notice of the superficial observer, hence Eels have been 
umproperly included among the Fishes forbidden as food by 
the Mosaic prescriptions. In the other genera, including the 
exclusively marine Conger of our coasts, scales are really absent. 
The Common Eel (Anguilla vulgaris) has a very wide dis- 
tribution, being found over the greater part of Europe, North 
Africa, Temperate Asia, and perhaps also North America east of 
the Rocky Mountains, Mexico, and the West Indies. Its record 
from Australia and New Zealand is probably due to the imperfec- 
tion of our knowledge of the specific characters. It is not found 
in the Black Sea nor in the rivers flowing into it, owing, no doubt, 
to the sulphurous nature of the bottom of the sea, to which, as 
we now know, these Fish would have to resort for breeding. 
The mode of propagation of the Eel long remained a mystery, 
from the fact that individuals found in fresh water never 
show ripe genital glands. The idea had been entertained of their 
being hermaphrodite, and internal parasites had also given rise 
to the belief in their viviparous nature. The genital glands of 
the female were first investigated by Rathke in 1858, but it was 
not until 1874 that those of the male were discovered by Syrski, 
and shortly after fully described by L. Jacoby, who, in his final 
contribution to the subject, concluded that Eels need salt water 
for the development of their organs of generation, and that this 
