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 Amice and Lepidostei 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, Holoslei, 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 Amiad^e, 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 

 Butijrinus, if this be properly included. The title of Amia to a distinct 

 position, as the type of a family, among the GanoidcB, instead of forming, as 

 heretofore, one of the Clupeidce, is mainly to be found in the five or six 

 valves in the aorta. The Amiadce 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 Amia, 

 with the general characters already referred to, has the head exhibiting 

 conspicuous sutures ; a long dorsal and a short anal ; 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 Ajnias 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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