MULLER AND MIDDENDORFF ON FOSSIL FISH FROM SIBERIA. 47 



of the fish. The jaws are provided with very small, and often scarcely 

 visible teeth. The number of streaks on the skin by the gills seems 

 to be considerable ; in some specimens eight to twelve could be di- 

 stinctly seen, and there may have been more of them. The dorsal 

 fin is placed over the anal fin ; it commences behind the anal fin, and 

 has ten soft or articulated rays. The anal fin, of similar construction 

 and form, has fourteen. The ventral fins are abdominal, halfway be- 

 tween the root of the ventral fins and the anal fin. The tail or caudal 

 fin is forked, with equal upper and lower lobes. The ray-joints of 

 the caudal fin are three times as long as they are broad. There are 

 about forty or more vertebrae. When they are so displaced that the 

 sides of the joints are visible, they appear, as in the case of the 

 Thrissops of the lithographic slates under similar circumstances, like 

 rings filled up with stone. The end of the vertebral column rather 

 approaches the upper lobe of the caudal fin. There are twenty pairs 

 of ribs, all comparatively thin. The scales are round, extremely thin, 

 I- to J of a line in size, and show when magnified obscure concentric 

 lines on their upper surface. 



The size of the fish is 2 inches and upwards. In the shale are 

 impressions of much larger but similar fish, but of which the frag- 

 ments are too small to allow of any description. 



The Siberian fish belong to the order Physostomi of Milller. It is 

 impossible to give the family of the fish with any certainty, although 

 it is probable that they belong to the family of the Esoces. 



On one of the slabs of shale is the jaw of another fish, with com- 

 paratively long, pointed, and curved teeth, represented in pi. 11, 

 fig. 3, and magnified twice the natural size. 



PI. 11, figs. 1, 2. Ly copter a Middendo7-ffii, twice the natural size. 



Appendix respecting the locality where the Fish-impressions were 

 found. By Herr v. Middendorff. 



By some mistake Prof. Miiller appears not to have received a 

 notice I had prepared respecting the locality of the above-described 

 fossil fish, I therefore insert it as an appendix. 



These fish-impressions are the same as those to which I alluded in 

 my last notice from Siberia, addressed to the Academy (Bulletin, p. 

 14), and for which I am indebted to the zeal of Mr. N. A. Sensinow. 

 Mr. Sensinow gave me the following information respecting their 

 site : — About 140 to 150 wersts S. of Nertschinsk, and distant about 

 70 wersts from the nearest point of the Chinese frontier, the little 

 river Byrka (probably the Mongolian name for an inapproachable 

 spot) falls on the right bank into the Turga, and about 40 wersts 

 above the spot where the Turga itself falls into the Onon, also from 

 the right bank. 



Upwards from the above-mentioned mouth of the Byrka, a slaty 

 clay forms the right bank of the Turga, which has cut for itself a 

 deep and narrow bed in the clay. About six feet below the surface 

 of the slaty clay, and in the clay itself, are found the impressions 

 above described by Prof. Miiller. Fish and shells all lie together, 



e 2 



