236 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 inthe 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 beensfound in North 
America. 
The family of the Leprottr embraces species of slender form ath 
powerful build, whose anterior fin-edge is provided with a double series of 
fulera, 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 first present themselves in the Jura, and disap- 
pear at the beginning of the Tertiary. None belong to North America. 
In the family Monosticum the anterior fin rays are provided with a simple 
series of fulecra, 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 equilaterally rhomboidal, or losenatseee lara 
‘The teeth are acutely conical, or bluntly cylindrical. 
The Dierertni Homocerctare distinguished from all the preceding families by 
the two dorsal fins, and from the ie ee 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. 
Dirrerini HeTERocEeRcr 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 
ef fulcra, sustains small close pressed jointed rays, which sometimes form 
alone the upper lobe of the caudal fin. The body is generaily elongated, 
and the jaws armed with a series of conical teeth, of equal or unequal size. 
The thin, scales pass from rhomboidal to cyeJoidal in shape, in which latter. 
‘case they are imbricated over the body. From the configuration of head, 
they exhibit a remarkable approximation to the living ganoids. 
In the family of Acanruoptr 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 
elongated spine, with neither fulcra nor jointed rays, thus distinguishing the 
genera of this family from all other ganoids. The exceedingly small scales 
440 
