430 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. V. 



There are but two living genera, namely, the Lepidosteus, 

 of which several species inhabit the rivers of America ; and 

 the Polypterus, that comprises two species, one inhabiting the 

 Nile, and the other the rivers of Senegal. In these fishes the 

 bones of the skull are closely connected by sutures ; the teeth 

 are large, conical, and longitudinally striated, as in the croco- 

 dile; the spinous processes are united to the bodies of the 

 vertebrae by suture, as in most reptiles; and the ribs are 

 articulated to the extremities of the transverse processes ; the 

 skeleton is osseous; the scales are flat, rhomboidal, and 

 parallel to the body. Even in the soft parts many analogies 

 are presented ; thus the Lepidosteus has a glottis, as in the 

 Siren, and a cellular air-bladder, with a tracheal vessel, re- 

 sembling the lungs of an Ophidian (serpent). These fishes 

 are the only living representatives of those voracious tribes of 

 the ancient marine faunas, whose remains abound in the 

 secondary formations. 



The fossil remains of the fishes of this family have often 

 been mistaken for those of reptiles, particularly the teeth, 

 which from their large size, conical figure, enamelled and 

 striated surface, and internal cavity, were supposed to belong 

 to crocodiles. 1 These teeth consist of two kinds : the outer, 

 or fish-like system, consisting of numerous small brush-teeth ; 

 and an inner row of large, pointed, conical, striated, enamelled 

 teeth, placed at a distance from each other, as seen in the fine 

 jaws of Rhizodus in No. 9, of Case B, (p. 433.) 2 



The sauroids, like the lepidoids, form two groups : 1, the 

 homercercals contain fourteen or fifteen genera, among which 

 are the Leptolepis, Aspidorhynchus, and Belonostomus ; 2, the 

 heterocercal tribe, which includes ten or twelve genera, 

 and among them some of the largest and most remarkable 

 ichthyolites of the Carboniferous system, viz. the Rhizodus, 

 Megalidiihys, and Saurichthys. 



Leptolepis. Wall-case B. (No. 7.) The small fossil fishes, 

 resembling a fry of Herrings, in the white lias of Solen- 



1 An interesting paper, " On the Microscopic Structure of the Teeth 

 of the Lepidostei, and their analogy with those of the Labyrinthodonts, 

 with a Plate," by Dr. Jeffries Wyman, will be found in " Amer. Journ. 

 of Science," October, 1843, p. 359. 



2 See " Medals of Creation," p. 651. 



