Recherches sur les Poissons Fossiles. 



327 



cation advances in this way, and the scales appear in straight 

 lines, it produces sometimes a longitudinal series of scales, 

 to which we give the name of lateral line, which is found to 

 be composed almost entirely in this manner, as in the Pike, 

 the Coregoncs, etc. 



Numerous modifications in the external aspect of scales 

 result from the form of their imbrication, depending on the 

 degree of alternation of the scales in their successive dorso- 

 ventral series ; for, according to the inclination of these se- 

 ries, a superior scale of any one series may be sufficient 

 to cover simultaneously with an inferior scale of the pre- 

 ceding series a great portion of the two sides of the adjoin- 

 ing scales, according to the degree in which they are drawn 

 together ; for example, as seen in the Leuciscs and our 

 Perches. The scales which are not imbricated, are either 

 very small and plunged into the skin in a manner to render 

 them imperceptible to the naked eye, or they assume the 

 form of shields or darts, sometimes standing equally over 

 the whole surface of the body (the Diodons); sometimes 

 they are overlapped diagonally or in a tesselate form, (the 

 Coffres ;) or finally, they form peculiar series in certain parts 

 of the body, while the rest of the surface is furnished with 

 other scales, (the Sturgeons, the Raies, etc.) 



The form of scales depends also greatly on their posi- 

 tion ; it is necessary therefore to attend particularly to the 

 position of the series from which they have been taken, 

 especially with regard to imbricated scales. These series 

 are disposed obliquely one behind the other, from the ridge 

 of the back to the middle of the belly. To this series we 

 apply the term dorso-ventral series. Generally, in each of 

 these the dorsal scales are smallest, those of the middle and 

 flanks are the largest, then they diminish again in size, and 

 it is on the lower parts of the body that they are observed 

 the smallest of all ; but there are exceptions to this general 

 disposition, and sometimes the lowest scales are the largest. 



