THE GANOIDEI. 137 



of the bod)', the anterior ends of the pectoral fins not con- 

 nected with the skiiU by cartilages, and the skuU with a 

 median facet for the first vertebra; and the Rays (Bajce), 

 with the branchial clefts on the tinder-surface of the body, 

 the pectoral fins united by cartilages to the skull, and no 

 median articiilar facet upon the occiput for the first vertebra. 



The Elasmabranchii are essentially marine in their habits ; 

 though Sharks are said to occur very high up in some of 

 the great rivers of South America. 



Both divisions of the Plagiostomi occur in the Mesozoic 

 rocks. In the Palaeozoic epoch, dermal defences and teeth 

 of Elasmobranchii abound in the Permian and Carboni- 

 ferous formations, and are met with in the Upper Silurian 

 rocks. But, except in the case of Pleuracanthus (a Selachian ). 

 it is impossible to be certain to what special divisions they 

 belong. 



IV. The Ganoidei. — In former periods of the world's 

 history this was one of the largest and most important of 

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

 seven genera — Lepidosteus, Polyptenis, Calamoichthys, Amia. 

 Accipenser, Scapirhynchus, and Spatularia, which are either 

 partially, or wholly, confined to fresh water, and are found 

 only in the northern hemisphere. These fishes diifer very 

 widely from one another in many points of their organiza- 

 tion, but agree in the following characters, some of which 

 they possess in common with the Elasmobranchii, and 

 others with the Teleostei. Thus : — 



a. The bidbtts aortoe is rhythmically contractile, and pro- 

 vided with several rows of valves, as in the Elasmobranchii. 



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

 hranchii. 



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

 as in the Elasmohmnchii, 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, which extends beyond 



