GENERAL INFERENCES. 79 



Pteraspis,"* says that ganoid fislies are distinguished from all 

 others by certain peciiharities which are connected with the form 

 of the commissure of the optic nerves, the aorta, ])ranchise, the 

 intestines, and the ventral fins. " These essential characters," as 

 described in the paper, " are shared by only six genera of existing 

 fishes, Lepidosteus, Polypterus, Amia, Acipenser, Scapirhynchus, 

 and Spatularia, which are no less singular in their distribution 

 than in their anatomy. All are e^^e;ni\?i\\.y fresh-ivater fishes ; all 

 are found in the northern hemisphere ; three — Lepidosteus, 

 Amia, and Spatularia, are exclusively North American ; Poly- 

 pterus is only known in the rivers of Africa, while Acipenser is 

 common to Europe, Asia, and North America," The palseon- 

 tological ganoids are no less extensively distributed, prevailing 

 wherever the Old Red Sandstone is found, all over the north of 

 Europe and America, the central regions of Asia, and other 

 quarters of the globe. 



But, on the other hand, says Mr. Page,f looking at the whole 

 s^^stem of the Old Red Sandstone, both in point of time and 

 composition, we are prominently reminded of marine condi- 

 tions — " of sea-shores whose sands formed sandstones, and of 

 beaches whose gravel was consolidated into conglomerates and 

 puddingstone — of receding tides that produced ripple-marks, 

 and of showers that left their impressions on the half-dried 

 silt of muddy estuaries." Shall we add, that the vast and 

 world-wide geographical extent of these deposits confirm the 

 marine theory of their origin, since we can scarcely suppose 

 fresh-water lakes of co-ordinate dimensions at any time existing 

 on the face of the planet % But, again, when we examine the 

 fossil vegetable remains enclosed in the strata, and these most 

 abundant not among the littoral grits and breccia, but in the 

 tilestones and finest micaceous beds inferring deep-water for- 

 mation, we are constrained to recognise the existence of marshes 

 and river-banks which gave birth to a growth of ferns, trees, 

 and shrub-like plants ; and of estuaries and fenny lagoons, 



* Quart. Joiirn. Geol. vol. iv. p. 279. f Advanced Tfxt-Boolc of Geology, p. 133. 



