OF THE BRITISH ISLANDS. 105 



ORDER ll.—Ganoidel 



Skeleton partly cartilaginous, partly ossified ; the optic 

 nerves forming a chiasma, not decussating ; the aortic bulb 

 provided with but a single row of valves ; the intestine 

 with a spiral valve ; branchiae free ; the gill cavity covered 

 by a gill-cover. 



FAMILY I. — Sturgeon Tribe (Acipenseridce). 



Skeleton partly cartilaginous ; the integument naked, 

 or protected by osseous bucklers ; the caudal fin un- 

 symmetrical, heterocereal ; the snout produced above and in 

 front of the mouth, four barbels disposed in a transverse 

 row developed from its lower side ; the mouth small, 

 toothless, highly protractile ; air-bladder large, communi- 

 cating with the dorsal wall of the sesophagus. 



The Broad-nosed Sturgeon (Acipenser maculosus), No. 

 195, and the Common Sturgeon {A. sturio), No. 196, are 

 the only British examples of the Ganoid fishes, represented 

 at the present day by some half-a-dozen remarkable exotic 

 genera, but which in older Geological times were among 

 the most abundant of the finny tribes. The Sturgeons 

 are either exclusively inhabitants of fresh water or migrate 

 periodically from the sea into the larger rivers to deposi t 

 their spawn. Both of the above-named species attain to a 

 large size, a length of eight or ten feet being an ordinary 

 measurement of the common sort, while the broad-nosed 

 variety is stated to grow to over twice these dimensions. 

 The flesh of the Sturgeon is much esteemed by some as an 

 article of food. In Russia, where the two British and other 

 allied forms are so abundant as to constitute a most 



