40 FOSSIL FISHES. 



of the exclusively freshwater forms have at present been met with 

 in a fossil state. 



The genus Osmerus, commonly known as the " Smelt " (Osmerus 

 eperlanus), is met with in considerable numbers in many localities 

 along the coasts of Northern Europe and America. It ascends the 

 tideway of rivers for the purpose of spawning. In the sea it attaius 

 a length of eight inches ; but it frequently migrates from the sea 

 into rivers and lakes, where its growth is very much retarded. 



That this habit is one of very old date is proved from the fact 

 that this small freshwater form occurs and is fully acclimatized in 

 lakes which have now no open communication with the sea. And 

 still more remarkable, this same migratory habit, with the same 

 result, has been observed in the Smelt of New Zealand (Retropinna 

 Michardsoni) . 



Osmerus also occurs fossil in the Greensand of Ibbenburen, in 

 the Eocene slates of Glaris and the Miocene of Licata. 



The genus Osmeroides, of which there is a fine series in the col- 

 lection, consists of fishes closely allied to the " Smelts." Their 

 remains are not uncommon in the English Chalk, in reference to 

 which we may here remark that many of the Chalk Ichthyolites 

 occur uncompressed, the result of calcareous particles replacing the 

 muscular and other tissues as rapidly as they were destroyed by 

 decomposition, and thus preventing the collapse of the flanks, and 

 preserving the natural rotundity and form of the fish when living. 

 Many illustrative examples may be seen in the cases ; and notably a 

 specimen of this genus that has frequently been figured, and which 

 Dr. Mantell, who extracted it from the chalk, thus describes : — 

 " This matchless ichthyolite is nine inches long, and the chalk has 

 been cleared away so as to expose the entire body, lying six inches 

 in relief above the block, to which it is attached by the dorsal 

 aspect. The mouth of the fish is open, the opercula or gill covers, 

 and the branchial arches are expanded ; the pectoral and ventral 

 fins, and the dorsal fin, are in their natural position."* 



Acror/natlnis is an extremely rare fish, and Aulolepis (Pipe-scale) 

 is far from being common. It derives its name from the peculiar 

 form of the tube in the scales of the lateral line. This line is present 

 in some form in all the osseous fishes, both fossil and recent, and 

 consists of a successional series of perforated scales, extending from 

 the head to the tail on each side; their function being the excretion 

 of a lubricating mucus, which enables the fish to move rapidly 

 through the water without friction. 



The genera Osmeroides, Acrognathus, and Aulolepis have been 

 found fossil in the Chalk of Lewes. 



The " Capelin," Mallotus villosus,\s found in great abundance on 

 the Arctic coasts of America aud Ivamtschatka. It is caught in 

 immense numbers by the natives, who consume it fresh, and also 



* " Petrifactions and their Teachings" (p. 449), by G. A. Mantell, M.D., F.R.S., 

 &c. 



