510 PISCES SUB-CLASS IILTELEOSTOMI. 



heterocercal type. In the skeleton, the vertebrae are unique among living 

 fishes in that the articular surfaces of their bodies form a cup behind and a 

 ball in front. The air-bladder has a duct, and the nerves supplying the eyes 

 where they meet one another in the middle line give off interlacing fibres, and 

 there are remnants of a spiral valve in the lining membrane of the intestine. 

 Bony pike, which are abundant in many of the North American rivers, are 

 very analogous in their habits to true pike, feeding upon other fishes. 



SUB-ORDER VII. PROTOSPONDYLI. 



The single existing family (Amiidce) of this once abundant subordinal 

 group is now represented by the bow-fin (Amia-calva) of the fresh waters of 

 the United States. This fish, while agreeing with the bony pike in the con- 

 formation of the optic nerves supplying the eyes, differs in the fuller develop- 

 ment of the spiral valve in the intestine, and in the simpler form of the 

 bodies of the vertebrae, which are disc-like. Although the scales are coated 

 with ganoin, they differ from those of the bony pike in being thin, rounded, 



and deeply over-lapping. A peculiar 

 feature of the bow-fin, and the one 

 from which it takes its vernacular 

 name, is the great length of the 

 dorsal fin, which occupies more than 

 half the entire length of the back, 

 and posteriorly is separated only by 

 a notch from the tail-fin ; the latter 

 Fig. 33. BOW-FIN. being of the abbreviate heterocercal 



type. The bow-fin grows to a length 



of about a couple of feet, and is a carnivorous fish, exceedingly abundant in 

 some of the North American lakes. It feeds not only on other fishes, but 

 likewise on various invertebrate animals, and can exist for a considerable 

 time out of water. Its favourite haunts are the dense masses of floating 

 vegetation fringing the North American lakes, among which it lays during 

 the month of May a host of minute eggs. 



SUB-ORDER VIII. CHONDROSTEI STUEGEONS. 



The title of royal fish bestowed on the common sturgeon would seem to in- 

 dicate that it has some special claim to superiority over the other members 

 of the finny tribe. This, however, is not the case, the title being due to the 

 circumstance that all such fish, according to an act of Edward II. which 

 appears to be still in force, belong of right to the crown. The enactment 

 runs that " the king shall have wreck of the sea throughout the realm, whales, 

 and great sturgeons, taken in the sea or elsewhere in the realm, except in 

 certain places privileged by the king." From all the other subordinal groups 

 of the Teleostomi, the sturgeons and their extinct allies are broadly distin- 

 guished by the greater relative number of the dermal rays in the dorsal and 

 anal tins, which exceed the bony or cartilaginous elements pertaining to the 

 true skeleton by which they are supported, instead of these two elements 

 being numerically equal. As regards the optic nerves and the lining mem- 

 brane of the intestine, the sturgeons resemble the preceding sub-order. All 

 the members of the sub-order have a persistent notochord and a cartilaginous 

 skeleton, but the tail may be of either the heterocercal or the diphycerca, 



