46 GEOLOGICAL MEMOIRS. 



sions sent to me by ^l. t. ]Middendorff, is one containing a specimen 

 of tliis mollusk. Dr. Troscliel^ to Trhom I submitted it, decided 

 that tlie shell in question could not be distinguished from Paludina. 

 Under these circumstances it may be assumed as certain that the 

 beds in question belong to a freshwater formation, and it is at the 

 same time highly probable that they are tertiaiy. 



This conclusion is the more satisfactory, as the fish-impressions 

 themselves are not of such a nature as to give any certain data re- 

 specting the age of the foiTiiation. It is true that the character of 

 the fish in the above remains at once reminds us of the fish of the 

 class Teleostei, which first make their appearance in the history of 

 creation in the chalk, and are the prevaihng forms amongst those 

 now lining as well as in the tertiaiy formations. At the same time 

 the fish of these shales have some resemblance to certain species 

 of Thrissojps which occur in the hthographic shales, particularly to 

 the Thrissops ce])lialus of Agassiz, at least in the position of the fins, 

 the great number of gill-rays, in the structure and number of the ver- 

 tebrae, and in the scales. This resemblance has been already noticed 

 by Count Keyserling. The genus Thrissops, which belongs to the 

 Ganoid family, is only shghtly characteristic of the Ganoid form ; so 

 little indeed, that in their general features they are closely allied to 

 the forms of the Teleostei, which have not yet been recognized in the 

 hthographic slates. 



If we had not possessed the above-mentioned proofs, which mi- 

 equivocally show that these are freshwater shales and of the tertiary 

 age, we might have been led by the resemblance of our fish-impres- 

 sions to the Thrissops cephalus to regard them as older. But as the 

 Thrisso2:)s has not been found in any fonnations newer than the litho- 

 graphic shales, we must be careful not to attach too much importance 

 to the analogy of form and structm-e of the fins, and it is therefore 

 deshable to estabhsh a new generic name for the Siberian fish, which 

 are undoubtedly Teleostei. 



It would also appear that the genus Thrissojjs should be subdivided 

 into two distinct genera ; for the Thrissops cephalus of Agassiz and 

 a few other species, not possessing ossa interspinosa on the finless 

 portion of the back, do not appear to belong to the same genus 

 Thrissops which contains species with ossa inferspi?wsa on that por- 

 tion of the back, and which, as it appears, have also fulcra for the 

 fins, a feature possessed neither by the Thrissops cephalus nor by 

 the small fish discovered by Herr v. INIiddendorff. 



^"hatever name may be given to the Thrissops cephalus and its 

 allied species which must be separated from Thnssops, there are very 

 sufficient reasons for not uniting them T\-ith the genus of the Siberian 

 fish. I must not omit to observe, that amongst the genera of living 

 fish there is none to which the Siberian fossil fish can be referred. 

 In order to designate this genus I have chosen the name Lycoptera, 

 to signify that the fins are placed in the same way as in the Pike. 



Lycoptera Middendorffii, Miill. 

 Descriptio7i. — The head is equal to one-fourth of the whole length 



" 



