THE GANOIDEI. 121 



formations, and are met with in the Upper Silurian rocks. 

 But, except in the case of JPleuracanthus (a Selachian), it is 

 impossible to be certain to what special divisions they belong. 



lY. The Ganoidei. — In former periods of the world's his- 

 tory, this was one of the largest and most important of the 

 orders of fishes; but, at present, it comprises only the seven 

 genera — •Lepidosteus, Polypterus^ Calamoichthys, Ai7iia, Ac- 

 cipenser, Scapirhynchus, and Spatidaria, which are either par- 

 tially or wholly confined to fresh water, and are found only in 

 the northern hemisphere. These fishes differ very widely from 

 one another in many points of their organization, but agree in 

 the following characters, some of which they possess in com- 

 mon with the jElasmobranchii, and others with the Teleostei. 

 Thus : 



a. The hullnis aortce is rhythmically contractile, and pro- 

 vided with several rows of valves, as in the JElasmobranchii, 



b. The optic nerves unite in a chiasma, as in the JElastno- 

 branchii. 



c. There is a well-developed spiral valve in the intestine, 

 as in the JE lasmobranchii^ in all but Lepidosteus, which pos- 

 sesses only a rudiment of such a valve. 



On the other hand : 



a. The branchial processes are not fixed throughout their 

 extent to the wall of a branchial sac, Avhich extends beyond 

 them, as in the Elasmobranchii / but their extremities project 

 freely beyond the edge of the septum which separates each 

 pair of branchial clefts, as in the Teleostei ; and, as in the 

 Teleostei^ they are covered by a bony operculum. 



b. There is a large air-bladder connected by a permanently 

 Qi^Qw. pneumatic duct with the oesophagus, as in many Teleostei. 



c. As in the Teleostei^ there is no cloaca. 



The ventral fins are always abdominal in position. The 

 tail is diphycercal, or heterocercal, and the terminal portion 

 of the notochord is not ossified. The cavity of the abdomen 

 is placed in communication with the exterior by abdominal 

 pores. Finally, the ducts of the reproductive organs communi- 

 cate with those of the permanent urinary apparatus, which is, 

 in part, an Elasmobranch, in part, an Amphibian, character. 



The exoskeleton presents the most extreme variations in 

 the Ganoidei. Spatularia is naked ; Accipenser and Scapi- 

 rhynchus develop numerous dermal plates composed of true 

 bone ; Amia is covered with overlapping cycloid scales ; Lepi' 

 dosteus and Polypterus have solid, rhomboidal, enamelled scales, 

 6 



