356 VERTEBRATES: FISHES. 



Linn., of Europe, attains a weight of twelve or fifteen 

 hundred pounds. The isinglass of commerce is prepared 

 from the swimming bladder of the Sturgeon. 



SUB-SECTION VIII. 



THE ORDER OF PLAGIOSTOMI, OR SELACHIANS. 



THIS Order and the next comprise cartilaginous fishes 

 called Fixed-Gilled Chondropterygians. Instead of hav- 

 ing the gills free on the external edge, and opening at 

 their intervals into a common chamber, as in all the 

 preceding fishes, these have them adhering by the exter- 

 nal edge in such a manner that the water escapes through 

 as many holes in the skin as there are intervals between 

 the gills ; or else the holes terminate in a common duct, 

 through which the water passes out. 



SQUALID^:, OR SHARK FAMILY. This Family em- 

 braces the Sharks, several species of which are viviparous, 

 and others produce eggs invested with a yellowish trans- 

 parent horny substance, the angles of which are pro- 

 longed into horny tubes. 



The Genus Alopias contains the Long-tailed or Thresh- 



Fig. 237. 



Thresher Shark, A . vulpes, Bon. 



er Shark, A. vulpes, Bon., twelve to fifteen feet long, with 

 the upper lobe of the tail about the length of the body. 

 It inhabits the Atlantic. 



The Genus Lamna has the snout pyramidal, with the 

 nostrils under the base. The Mackerel Shark, or Green- 



