H20 Recherches sur les Poissons Fossiles. 



which are attributed different functions; for example, the 

 teeth and all the horny plates, which in many classes are 

 found on the internal surface of the intestines. Two essential 

 modifications of the skin may therefore be distinguished, and 

 consequently also, two modifications of the dermatic skeleton. 

 The one which covers the external surface of animals, and 

 the other which is developed in their internal surface. 

 These two kinds of skeletons exist together in the inverte- 

 brated animals, and present betw r een themselves very inti- 

 mate and numerous relations and connexions, as we shall 

 presently see ; they are seen also in many places passing in- 

 sensibly into one or other at the superficial openings of the 

 internal cavities of the body. They exist also constantly 

 double in all the vertebrate animals, which have in other res- 

 pects an interior bony structure encircling the cavities, and 

 round which all the organs are placed. 



In this great division of the Animal Kingdom, not only 

 do the two modifications of the dermatic skeleton present 

 numerous connexions, but these again afford intimate rela- 

 tions to the bony skeletons on many parts of the body, 

 where insensible transitions are observed from one to the 

 other; for example, in the fishes, between the opercular 

 pieces and the scales, between the occipital bone, the 

 humerus and the scales, between the teeth and the pha- 

 ryngeal bones, &c. &c. 



It exists besides in a constant antogonism in the develop- 

 ment of the three kinds of skeletons I am describing, and of 

 which the parts of the one are increased in proportion 

 as those of the other are less complete in different regions 

 of the body. 



No one has yet obtained better information of the dif- 

 ferent modifications of the skeleton than Carus ; no one has 

 examined it in more detail ; but no one has explained it in a 

 manner more confused than he has done in his work, on the 

 essential parts of the osseous and testaceous structure. 



