FISHES OF LAKE SUPERIOR. 267 



an opportunity in former years of studying almost all the European 

 species in a fresh condition, during a prolonged visit in Vienna. 



ACIPENSER L^BVIS, Agass. 



This species, one of the largest of the genus, is from the Pic. 

 The length of the specimen, of which I possess the head and the 

 fins, and which was in fresh condition when I examined it, was four 

 feet six inches. The head, which is contained two and a half times in 

 the whole length, is subconical and a little flattened below ; the upper 

 surface forms an uniformly descending line from the occiput to the ex- 

 tremity of the snout, somewhat elliptical beyond the eyes, thus giving 

 to the latter a slightly recurved appeai^ance. From the level of the 

 eyes to the centre of the skull, on the middle line of the head, there 

 exists an equally elongated surface, more flattened, being the rudi- 

 ment of a longitudinal dimple ; finally, on the occipital part of the skull 

 we observe a small keel, where the two bones of this region begin 

 to become convex, in order to pass to the cutting plates of the back. 



The surface of the bones which form the exterior covering of the 

 head, is invested with small tubercles of enamel, of a circular form 

 with obtuse summits. At first without apparent order, at the very 

 centre of the bone they become linear, radiating to the circumfer- 

 ence. Their greatest development occurs in the occipital region and 

 on the transverse line level with the nostrils. On the middle part 

 of the head these tubercles become thinner, and on the extremity of 

 the snout they are reduced to a fine reticulation. The sides of the 

 head have only a very few asperities. The only bone on which they 

 are developed is the operculum, and it is only in its posterior half 

 that they radiate from the centre towards the margin. A few rows 

 only are directed towards the upper part of the head. The other 

 bones constituting the opercular apparatus are covered with a 

 membrane finely roughed at the surface. The bones placed at 

 a small distance behind the eye and limiting the anterior margin of 

 the branchial cavity, bear a few blunt tubercles irregularly dis- 

 tributed on their surface. The branchiostegal membrane is naked 

 and smooth, attached by a thin shred to the posterior part of the 

 operculum, and passes before the pectoral fin, to which it is con- 



