FISHES. 227 



is avoided by the teeth not being fixed in sockets, but attached 

 to a cartilaginous membrane. Tlie first row of teeth stands 

 erect, the others are laid flat behind. The membrane con- 

 tinues to grow, and advance forward, the outer teeth drop out, 

 the membrane itself is thrown otF or absorbed, and the row 

 which was originally second takes the place of the first, all the 

 teeth in it standing erect, until, in the course of time, they make 

 wav for a third series, which is followed by others in succession. 

 Sturioxid.t;. — The only remainmg fish we shall mention 

 belonging to the cartilaguious group is the Sturgeon (Acipenser 

 Stitrio, Fig. 19S), and it approaches to the other lamilies of 



Fig. 19S.— Sturgeon. 



fishes in being oviparous, and in having the gills free. Its ex- 

 ternal appearance is striking, and the series of bony plates 

 upon the surface of the skin is very remarkable. 



In comparing the figures of the Sturgeon (Fig. 198), and of 

 the Dog-fish (Fig. 196), with that of the Perch (Fig. 181), 

 the appearance presented by the tail is extremely chfierent. 

 In the perch, the vertebral column ceases at the tail-fin, which 

 if the line of that colimm were continued, would be divided 

 by it into two equal parts. In the Sturgeon and others, the 

 vertebral column is continued into the upper portion of that fin, 

 and symmetrical appearance in the organ is therefore wanting. 

 This is one of the obvious external characters by which the 

 cartilaginous fishes may be distinguished from the osseous. In 

 remote periods of the earth's history, this peculiarity of struc- 

 tm-e appears to have prevailed universally : it is found in every 

 fossil fish whose remains are preserved in the magnesian lime- 

 stone, and in strata of older formation. 



The Sturgeon, when caught in the Thames, within the juris- 

 diction of the Lord Ma^'or, is considered a royal fish ; the 

 term being intended to imply that it ought to be sent to the 

 king.* One taken in 1833, in Scotland, measured eight feet 

 six inches in length, and weighed 203 lbs. Pennant mentions 

 * YarrcU, vol. ii. p, 362. 

 Q 



