52 Fossil Fishes'. 



logical observations on the contemporaneous formation of masses of 

 strata, p. 188. 2. D. granulatus, the rarest species of the genus 

 found at Lyme-Regis. 3. D. punctatus, is peculiar to the lias of 

 Lyme-Regis. 4. D. Colei, Lyme-Regis. Of the genus Tetrago- 

 nolepis, the following British species are described: — 1. T. conflu- 

 ens, in the fine collection of Lord Cole, from Lyme-Regis. 2. T. 

 speciosus, in the collection of Lord Cole, from Lyme-Regis. 



But we must conclude, for the present, our extracts from Agas- 

 siz' work, with the following curious observations connected with the 





genera Cephalaspis, Dipterus, Osteolepis, Acanthoides, Cheiracan- 

 thus, Cheirolepis, Amblypterus, Gyrolepis, Palseoniscus, Platysomus, 

 and Eurynotus, in all of which the tail is unsymmetric, having the one 

 lobe longer than the other. 



" There is one very remarkable fact concerning the relation of 

 these genera with the geological formations which they characterize ; 

 it is this, that all the known species, without exception, have been 

 found in the formations which are anterior to the formation of the 

 lias. Nor can this circumstance be accidental, it is again observed, 

 within the same kind of limits, and upon almost an equal number of 

 species in the Sauridian family ; at the same time that all the fish of 

 the Placoidean order, which accompany them in the same formations, 

 have also the same kind of structure of the tail. Some unknown 

 condition of existence, therefore must have been operating in those 

 remote times upon the development of organic life, and must have 

 effected a conformation which is at once so peculiar and so general : 

 For we are not permitted to regard phenomena so constant as these 

 as simple exceptions, since nature in her doings, never admits them 

 on so extended a scale. We can only regard these forms as neces- 

 sary antecedents of those which have followed ; and the traits which 

 characterize and distinguish them as the differences in the progress- 

 ive developments. These differences consist especially in the 

 transition of an unsymmetric structure to a structure more and more 

 perfect in its symmetry, which has gradually prevailed in subsequent 

 epochs, in which the unsymmetric forms have successively disap- 

 peared. To attempt to point out the causes of such a state of things, 

 would be to endeavor to fathom the motives of the Almighty Cre- 

 ator : yet at the same time we may venture to offer some conjectures 

 concerning the relations of the form of these fishes with the exter- 

 nal world in which they were destined to live. 



