234 ZOOLOGY. 
It has already been mentioned that most Ganoids are only known from 
their fossil remains. The living genera and species are few in number: 
the Amie and Lepidostet of North America, the Polypterus of Africa, and 
the Sturgeons of both hemispheres, being all that now exist. A convenient 
division of the Ganoids is into those with a bony skeleton, Holostei, and 
those with a cartilaginous skeleton, Chondrostei. In the Holostei, while 
the entirely bony character of the skeleton is the rule, yet an exception is 
found in some genera, in which ossified ribs and spinous processes are 
attached to a cartilaginous chorda. The ganoid scales are of various 
character, and in the first family the scales are even cycloid; other 
features, however, still retaining it among the Ganoids. A progression also 
is observable in the really Ganoid scale. At first it is rounded, and with a 
very slight coating of enamel; then this thickens, and the scale becomes 
more and more angular, still retaining the imbricated character. Finally, 
the scales become angular plates, in which a pin in the upper edge of one 
fits into a depression in the lower border of another immediately above, the 
whole thus riveted together, as it were, into a coat of mail. 
The first family of the Holostei is that of Am1apa, the type of which is 
Amia, a genus of fishes exclusively confined to North America. Most 
species of this family. as of most of the Holostei, are extinct, the recent 
being only those which belong to the above mentioned genus, and to 
Butyrinus, if this be properly included. The title of Ama to a distinct 
position, as the type of a family, among the Ganovda, instead of forming, as 
heretofore, one of the Clupeid@, is mainly to be found in the five or six 
valves in the aorta. The Amiade@ have an elongated, nearly cylindrical 
body, with a rounded or emarginated sub-homocercal caudal, one dorsal 
fin, variable in position, and flexible rounded, or subangular, mailed or 
imbricated scales. The jaws are provided with conical teeth, of greater or 
less size. The fishes of this family first appear in the Jura, occurring in 
small number in the Lias. Extending through the middle Jura, they 
disappear as fossil forms in the Cretaceous, leaving only Amia as their now 
living representative. None occur fossil in America, except a species of 
Aspidorhynchus, probably found in South America. The genus Amaia, 
with the general characters already referred to, has the head exhibiting 
conspicuous sutures; a long dorsal and a short ana]; a long buckler 
between the branches of the lower jaw; branchiostegal rays 12; conical 
teeth in the jaw, within which are smaller paved teeth. The head is short 
and rounded ; the nostrils have tubular appendages. The most conspicuous 
feature in the Amias is, however, to be found in the air-bladder, which is 
sub-divided into small cells exhibiting a structure very similar to that 
of some Reptilia. It is in this genus that the homology of the lung of 
the air-breathing vertebrate, with the air-bladder of the fish, is most clearly 
established. Amias, of which eight or ten species are known, all live more 
or less in the muddy bottoms of sluggish streams or ditches; and are 
generally shunned as repulsive objects unfit for food. They have been 
found throughout the United States, excepting in those rivers (and their 
tributaries) which empty into the Atlantic, between the St. Lawrence 
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