and on the Geological Distribution of Fossil Fishes. 177 



The species of the crag of Norfolk, the superior sub-apennine 

 formation^ and the molasse, are related for the most part to ge- 

 nera now common in tropical seas ; such are the piatax, the large 

 carcharias, the mjliobates, with large palatal plates, and others. 

 In the inferior tertiary formations, the London clay, the Calcaire 

 grossier of Paris and Monte Bolca, a third at least of the species 

 belong to genera which exist no longer. The chalk has more 

 than two-thirds of its species referable to genera which have now 

 entirely disappeared. In it we already even see some of those 

 singular forms which prevail in the Jurassic series. But, as a 

 whole, the fish of the chalk recall more forcibly the general cha- 

 racter of the tertiary fish, than that of the species of the Jurassic 

 series. 



If we paid attention only to fossil fish in the grouping of geo- 

 logical formations on a large scale, the author thinks it would be 

 more natural to associate the cretaceous with the tertiary strata, 

 than to place the former among the secondary groups. Below 

 the chalk there is not a single genus which contains recent species, 

 and even those of the chalk which have them, contain a much 

 greater proportion of species which are only fossil. The oolitic 

 series, to the lias inclusive, forms a very natural and well defined 

 group, in which also must be included the Wealden, in which 

 Mr Agassiz states he has not found a single species referable 

 even to the genera of the chalk. Henceforth, the two orders 

 which prevail in the present creation are found no more ; whilst 

 those which are in a small minority in our days, appear suddenly 

 in great numbers. Of the Ganoidians, those genera which 

 have a symmetrical caudal fin are found here; and among the 

 Placoidians, those above all predominate which have their teeth 

 furrowed on both the external and internal surface, and have 

 large thorny rays. For it is now certain that those great rays 

 which have beencalled Ichthyodorulites, belong neither to Silures 

 nor Balista?, but are the rays of the dorsal fin of the great squa- 

 loids, whose teeth arc found in the same strata. 



On leaving the lias to come to the inferior formations, we ob- 

 serve a great difference in the form of the posterior extremity of 

 the body in the Ganoidians. All have their vertebral column 

 prolonged at its extremity into a single lobe, which reaches to 

 the end of the caudal fin ; and this similarity extends even to 



vol.. XVIIT. NO. XXXV. JANU.ARY 1835. M 



