FISHES. 243 



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

 to a cartilaginous membrane. The 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 off 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 way for a third series, which is followed by others in 

 succession. 



STURIONID/E. The only remaining fish we shall mention 

 belonging to the cartilaginous group is the Sturgeon (Acci- 

 pensw Sturio, Fig. 198), and it approaches to the other 



Fig. 198. STURGEON. 



families of fishes in being oviparous, and in having the gills 

 free. Its external 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 different. 

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

 which, if the line of that column 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 there- 

 fore 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 structure appears to have prevailed universally: 

 it is found in every fossil fish whose remains are preserved in 

 the magnesian limestone, and in strata of older formation. 



The Sturgeon when caught in the Thames, within the 

 jurisdiction of the Lord Mayor, 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 2031bs. Pennant 



* Yarrell, vol. ii. p. 362. 



