OEDER VII. 



GANOIDEI. 



In the representatives of the Ganoid order the internal skeleton is either bony or else carti- 

 laginous, and the scales which invest the surface of their body consist either of angular or 

 rounded plates, of a bony base, and covered with an external layer of enamel, similar in structure 

 to the enamel of teeth, else they assume the shape of bony shields ; others still are perfectly 

 naked. The caudal fin is more or less heterocercal, and the anterior margin of the fins often 

 provided with a double series of shingle-like plates, or points the so called fulcrae. Several of 

 them are provided with an accessory breathing organ placed under the opercle, and which is to 

 be distinguished from a pseudo-branchia. Some of them are likewise possessed with spiracles. 

 The swimming or air bladder is provided with an air duct, which communicates with the throat, 

 as in many osseous fishes; but the rete mirahile is wanting. There are oviducts leading the 

 eggs out, without allowing them to fall in the abdominal cavity. Oftentimes the intestinal 

 canal exhibits a spiral valve, as in Plagios'omi. The manifold valves of the aorta, the free gills 

 protected by an opercle, an accessory gill under the gill covers, and the abdominal position of 

 the ventrals, are characters which the ganoids alone exhibit in combination. To these may be 

 added a peculiarity in the direction of the optic nerves, which consist in not crossing one another 

 before entering the orbit. 



SvN. — Ganoides, Agass. Rech. Poiss. foss. II, 1833, IX. 



Ganoidei, Bonap. Vert. Syst. 1837, 43 ,&, Catal. Pise. Europ. 1846, 4.— Agass. Nomencl. Zool. Pise, 1844. Mull. 

 in Wiegin. Archiv fQr Naturg. 1845, I, 129 &. 137. 



The structure and natural affinities of the fishes constituting the ganoid order have been made 

 the subject of very special investigations by Prof. Joh. Miiller, and which have somewhat modi- 

 fied their classification as formerly proposed by Prof. Agassiz. Midler's researches are the 

 standard of all future investigations upon this strange and curious order of fishes, and which 

 includes numerous extinct types, illustrated and described in the various works on Palaeontology 

 or fossil remains. 



Family AMIADAE, Baird. 



Opinions are at variance regarding the systematic position this family is to occupy in the class 

 of fishes. Its representatives are but few, perhaps reduced to a single living species, which has 

 been placed by some writers in the herring family, wliilst others have associated it with some 

 extinct types of past eras. Still others consider it as entitled to a place amongst Holostean 

 ganoids, on the ground of the aorta being provided with five or six valves. 

 Syk. — Jlmiadae, Bd Iconogr. Encyclop. II, 1850, 234. 



The genus Jmia, so far unique in this family, is exclusively confined to the fresh waters of 

 North America, and occurs chiefly in the southern and northwestern States, where they are 

 known under the names of "mud-fish" and "marsh-fish," and sometimes "dog-fish" also 



