230 ZOOLOGY. 



the caudal, are composed of weak rays. The ventrals are mostly wanting. 

 In addition to smaller teeth, variously situated, the lower jaw and the roof 

 of the mouth bear several series of broad depressed teeth, increasing in 

 extent, as they occur further back in the mouth. The surface of these teeth 

 is sometimes smooth, sometimes furrowed, and either flat, convex, or 

 concave. The roots of the teeth, which possess an internal cavity, are 

 firmly fixed to the jaw. There are strong sternal ribs, and peculiar slender 

 bones in the nape. The scales are of considerable size, of a thick rhom- 

 boidal shape. None of the Pycnodontes have as yet been found in North 

 America. 



The family of the Lepidotini embraces species of slender form and 

 powerful build, whose anterior fin-edge is provided with a double series of 

 fulcra, or imbricated scales, as in Lepidosteus. The inner dental series 

 sometimes have expanded obtuse teeth, the outer exhibits them conical and 

 slender. The caudal fin is always highly developed, attached obliquely, 

 and with the base of the upper lobe covered with small scales. The ventrals 

 are small, the pectorals well developed, and the dorsal of moderate size. 

 The genera of this family fifst present themselves in the Jura, and disap- 

 pear at the beginning of the Tertiary. None belong to North America. 



In the family Monostichii the antei'ior fin rays are provided with a simple 

 series of fulcra, which rest by two branches upon the main ray. The body 

 is more and more developed in a vertical direction. All the fins are feebly 

 developed, the caudal most so. The scales diminish remarkably in size, 

 from the sides, where they are greatest and higher than long, towards the 

 back, belly, and tail : becoming equilateral ly rhomboidal, or lozenge-shaped. 

 The teeth are acutely conical, or bluntly cylindrical. 



The DiPTERfNi HOMocERci are distinguished from all the preceding families by 

 the two dorsal fins, and from the succeeding by the homocercal or equilobed 

 character of the tail. This is generally highly developed at the expense of 

 the other fins, which appear small in proportion. The forked dorsal fin-rays 

 rest upon the intercalary spines, which in turn are supported immediately 

 by the spinous processes. The head is of moderate size, and the jaws 

 armed with strong, paved, or conical teeth. 



DiPTEUiNi iiETERocERci have the double dorsal of the preceding, and a 

 slightly developed, always heterocercal, or inequilobed tail. The bases of 

 all the fins are encompassed by smaller scales ; and the anterior ray, instead 

 of fulcra, sustains small close pressed jointed rays, which sometimes form 

 alone the upper lobe of the caudal fin. The body is generally elongated, 

 and the jaws armed with a series of conical teeth, of equal or unequal size. 

 The thin scales pass from rhomboidal to cyc'oidal in shape, in which latter 

 case they are imbricated over the body. From the confitruration of head, 

 they exhibit a remarkable approximation to the living ganoids. 



In the family of Acanthodii we find the same heterocercal tail as in the 

 preceding ; but the anal fin is simple, and the anterior dorsal soon disap- 

 pears. Both these median fins, as well as the lateral, have a first strong 

 elon'Tated spine, with neither fulcra nor jointed rays, thus distinguishing the 

 crenera of this family from all other ganoids. The exceedingly small scales 

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