318 Recherches sur les Poissons Fossiles. 



right to left, whatever may be the irregularity in the propor- 

 tions of one or other side of the animal, and it is upon these 

 proportions that symmetry depends. The height which here 

 is the thickness, is the differences which exist between the 

 dorsal and ventral regions, or according to the position of 

 the animal, between the superior and inferior parts ; essen- 

 tial differences which may be traced up from the separation 

 of the blastoderm into a serous and a mucous membrane 

 producing two orders of organs so different in the parts 

 which they are to perform in life. 



No other class of the animal kingdom presents these 

 forms in such variation as the class of fishes. There is no 

 other class in which such perfectly spheroidal animals as the 

 Diodons* occur ; other species are discoid, or circular, and 

 flat, but this last form presents two conditions very different 

 from the effects of either excessive contraction, or of exces- 

 sive development of the two sides of the body ; for in the 

 first case it is compressed, and very elevated, but also very 

 narrow, as in the Vomers, and the Orthogoriscs 5 whilst in the 

 second case it is very depressed, flat, and very broad as in 

 certain Rais. Other species are oval, more or less elongated, 

 slender, and compressed on the flanks ; this is the most 

 ordinary form in fishes, and hence those which possess it, 

 are called regular fishes, such as the Carps, the Trouts, &c, 

 yet notwithstanding the longitudinal direction varies con- 

 siderably, passing from every possible intermediate state, (in 

 the Pikes for example,) to the elongated fishes, which are 

 sometimes cylindric, (the Eels), sometimes compressed or flat 

 as a ribbon, (the Cepoles,) the same of others which have an 

 excessive breadth in comparison with their height, (the 

 Gymnetres and the Ophisures.) Although the forms most 

 fantastic, are those which present surfaces more or less 

 plane, and which are circumscribed by angular figures, 



* The Cutcutia (Tetradon cutcutia Buch.) is a familiar example of this 

 form in Bengal.— Ed. 



