M. Agassiz on Fossil Fishes. 346 



kingdom, the fossil species bear a greater resemblance to the 

 living species, in proportion as they belong to strata more 

 recent; and each new formation is a further approach to- 

 wards the actually existing state of things. The most im- 

 portant change in the entire class of fishes, has taken place at 

 the end of Jura epoch. Up to that period, all the fishes had 

 a peculiar physiognomy, in general very different from that 

 which we now perceive them to possess ; no other kinds were 

 to be met with but the Ganoid and the Placoid. It was not 

 till the time of the chalk formation that the two other orders, 

 the Ctenoid and the Cycloid, which almost exclusively prevail 

 in the present creation, made their appearance. The first 

 types of these orders belong, for the most part, to extinct ge- 

 nera, allied to our Clupea3 and Tunnies. In this epoch, fresh 

 water fish were still wanting. The fishes of the tertiary epoch 

 are much more nearly related to those of our own times ; a 

 great number belong to genera now existing : we find true 

 Tunnies, true Clupeae, true Anchovies, true Smelts, and fresh- 

 water fishes well characterised, such as Pikes, Leucisci, Tenches, 

 Loaches, Gudgeons, &c., but neither Trouts nor Salmon. On 

 the other hand, the Ganoids become more and more rare in the 

 tertiary formations. In a word, the Ichthyological Fauna of the 

 tertiary deposits, whether viewed as a whole or in its details, 

 presents the greatest analogy with that of our own times. In 

 order to shew more conspicuously the signification of these 

 difi'erent changes, the author has represented them, in a very 

 ingenious manner, in a pictorial sketch, which indicates the ap- 

 pearance of the different families, and their development, rela- 

 tively to the different eras. — {Plate 3d, contained in this 

 JSTo.) 



It is, in general, to the tertiary fossils that those geologists 

 who do not admit marked differences between the Faunas of 

 the different epochs, have recourse, in order to establish, ac- 

 cording to their views, the filiation of species across the dif- 

 ferent formations. They have even founded on the propor- 

 tional number of living species of mollusca, which they pre- 

 tend to have discovered in the strata of this period, a division 

 of the tertiary class into Eocene, Miocene, and Pliocene forma- 

 tions. Now, if these identities had been real, they ought to 



