534: 



FOSSIL FISH OF THE OLD RED SANDSTONE. [Oh. XXVI. 



of their scales. The same remark would hold true of the fish of the 

 primary and secondary formations generally, those of the primary and 

 older secondary type having heterocercal tails, while the tails of those of 

 the tertiary rocks are almost all equilobed or homocercal, like the vast ma- 

 jority of living fish ; but Prof. Huxley has also called our attention to the 

 fact that, while a few of the primary and the great majority of the sec- 

 ondary Ganoids resemble the living Lepidosteus, or bony pike, or the 

 Amia, genera now found in North American rivers, and one of them, 

 Lepidosteus, extending as far south as Guatemala, the Crossopterygii, 

 or fringe-finned Ichthyolites, of the Old Red are closely related to the 

 African Polypterus, which is represented by five or six species now 

 inhabiting the Nile and the rivers of Senegal. These North American 

 and African Ganoids are quite exceptional in the living creation ; they 

 are entirely confined to the northern hemisphere, unless some species 

 of Polypterus range to the south of the line in Africa ; and, out of 

 about 9000 living species of fish known to M. Giinther, and of which 

 more than 6000 are now preserved in the British Museum, they prob- 

 ably constitute no more than 27. 



All the living fish, exclusive of the 27 species just mentioned, and 

 the Elasmobranchii or Placoids, have equilobed or homocercal tails, 

 and are called Teleostei, because their skeletons are perfectly ossified.* 

 The living Ganoids, however, most resembling those of the primary 

 and secondary periods, namely, the Lepidostei and Polypteri, have also 

 internal skeletons as perfect as those of any Telostei ; and we find the 

 same combination of a hard external or dermal skeleton, and a well- 

 ossified endo-skeleton in Bipterus, one of the Old Red Ganoids already 

 alluded to. In this respect, therefore, Dipterus and Polypterus agree, 

 although they differ in their scales, Dipterus having cycloidal, and 



Polypterus rhomb oidal scales. Me- 

 ¥i s- coo. galiclithys, a carboniferous genus, 



agrees with Polypterus in the form 

 of its scales, which are rhomb oidal, 

 while its internal sketeton, as first 

 observed by Huxley, is so far ossi- 

 fied that in each vertebra there is 

 a ring or hoop of bone. 



The fossil Ganoids, therefore, al- 

 though generally contrasted with 

 the Teleostei, cannot be said to 

 have in all cases imperfect inter- 

 nal skeletons any more than the 

 most typical living representatives 

 of the order. 

 „,.*«'*. .., w . Amono* the anomalous forms of 



Ptertchthys, Agassiz ; upper side, showing » 



mouth ; as restored by h. Miller. Old Red fishes not referable to 



* From releog, teleos, perfect, and oareov, osteon, a bone. 



