M. Agassi z un Fishes. 345 



1. G. Albc7-tii, at Wick war, near Bristol. 2. G. tenuistriatiis, 

 in osseous breccia at Wickwar. 3. G. inaximus, at Wickwar ? 

 4. G. g'iganteus. The immense scales, found by Professor 

 Jameson and otliers in the old red sandstone of Fifeshire and 

 Angus-shire, belong to this species. Of the genus Dipedius, 

 the following are British species : — 1. D. politus, a species cha- 

 racteristic of the lias of Lyme-Regis : Then follows geological ob- 

 servations 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 Tetragonolepis, 

 the following British species are described: — L T. conjluens, 

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

 speclosus, in collection of Lord Cole, from Lyme-Regis. 



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

 Agassiz' work, with the following curious observations connected 

 with the genera Cephalaspis, Dipterus, Osteolepis, Acanthoides, 

 Cheiracanthus, Cheirolepis, Amblypterus, Gyrolepis, Palaeonis- 

 cus, Platysomus, and Eurynotus, in all of which the tail is un- 

 symmetric, 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 necessary 

 antecedents of those which have followed ; and the traits which cha- 

 racterize and distinguish them as the differences in the progressive 

 developments. These differences consist especially in the transition 

 of an unsymmetric structure to a structure more and more perfect 

 in its symmetry, which lias gradually prevailed in subsequent epochs, 

 in which the unsymmetric forms have successively disappeared. To 

 attempt to point out the causes of such a state of things, would be to 

 endeavour to fathom the motives of the Almighty Creator ; yet at 

 the same time we may venture to offer some conjectures concerning 

 thf relations of the form of these fishes with the external world in 

 which they were destined to live. 



