164> M. D'Orbigny on the Cotnparative Position of 



or according to zoological characters, as in the case of Deshayes ; but 

 without consultinor Nature to ascertain if the positions, thus arbitrarily 

 assigned, are in accordance with her. 



I have said that injurious consequences might result to science from 

 the representation of shells in a position contrary to nature, and I prove 

 my assertion in the following manner. In order to ascertain if strata 

 have undergone changes of position, if they have been formed at the 

 bottom of a basin, or on an ancient shore, geology and palaeontology 

 constantly require information, as to whether organic bodies, and espe- 

 cially acephala, or bivalve mollusca, are in their normal position, and as 

 to whether they have been rolled about or merely displaced. Now, hov,^ 

 can a geologist arrive at a conclusion on such points, by consulting, for ex- 

 ample, M. Deshayes' treatise on conchology ? As the plates in that work 

 represent bivalve shells in a position completely the reverse of their 

 normal one, he would naturally conclude that all the shells he meets with 

 in strata, have undergone a change of position, because there is no one 

 in the position assigned by the plates, whereas, on the contrary, these 

 shells may perhaps be in their normal state, as we find to be very fre- 

 quently the case. It is thus evident, that it is not at all a matter of 

 indifference to represent a shell in one way or another, and that it be- 

 comes indispensable for the zoologist, or the palaeontologist, to afford 

 geologists points of comparison upon which they may rely with certainty, 

 to enable them to ascertain the condition of the strata, at the moment 

 when the beings therein contained became covered by new deposits. 



I have already remarked, that there is a great difference between the 

 position of man, and that of ordinary quadrupeds. Now, we find another 

 example of such a difference in the position of fishes having a symmetri- 

 cal form, compared with that of the Pleuronectes, for the former are in a 

 vertical position, whereas the others are, relatively to them, placed on 

 the side. I insist on this last comparison of the position of fishes, be- 

 cause, in the bivalve acephala, we find precisely the same thing, as will 

 appear from the following observations. 



SYMMETRICAL SHELLS. 



Whenever a bivalve shell is perfectly symmetrical in its parts, so that it 

 is equivalve, we may assert a priori^ that its position is vertical or nearly 

 so, in the direction of the length. 



The genera Solen, Mya, Lutraria, Mycetopus, Panopcea, Sec, whose 

 form is the most elongated, are examples. Generally they are much 

 sunk either in the sand, or in the mud, where their tubes unceasingly 

 perform an up and down movement, in order to arrive at the surface, and 

 their position is perfectly perpendicular. 



When the shell, equally elongated, pierces a hole in a stone, as we see 

 in the genera Pholas, Lithodomus, Saxicava, Clavagella, Teredo, &c., the 

 shell is also perpendicular, the tubes above, the mouth below. 



When a free symmetrical shell is more or less oval or rounded, as is 



