On the Natural position of Shells. 207 



nate application of terms, of quite contrary signification, to the 

 same portions of the same shell. The part where the head of the 

 animal is situated, has thus been called the back, the base, and 

 the front ; a contradiction in terms arising entirely from a cul- 

 pable inattention to the natural position of the animal, from which 

 alone that of the shells can be correctly pointed out. 



In the symmetrical conical univalves, the front of the shell, or 

 that part over the head of the animal, is known by attending 

 to the fork of the muscular impression, the prongs of which are 

 directed forwards. The position of the apex or nucleus can not 

 be relied on for this purpose, as it is sometimes anterior, as in 

 Patella, and sometimes posterior as in Emarginula.* 



In the symmetrical convolute univalves, the front is in the centre 

 of the outer lip, that is to say on the opposite side from the volu- 

 tions; while in the spiral univalves, the front is always at that part 

 of the aperture, which is most distant from the apex of the spire, or, 

 in other words, at the base of the columella. That part of the aper- 

 ture next to the spire is consequently the back ; the inner, or colu- 

 mellar lip is the left side, and the outer, or marginal lip, is the 

 right side in all dextral shells ; the two latter being reversed in 

 all sinistral ones. 



The difficulties attending on the determination of the natural 

 position of the bivalve shells, have generally been considered as 

 much greater than those connected with the univalves, and hence 

 the contradictory application of terms has been in them more 

 frequent. It is indeed impossihle to understand the description 

 of a bivalve 8hell, without taking into consideration the particular 

 views of the author whose works we are engaged in consulting ; 

 Lamarck, for instance, generally terming that the front of a 

 shell, which should be called the back, and vice versa. Apply- 

 ing to these however the principle, that the mouth, or at least 

 the entrance to the digestive canal, is always to be regarded as 

 the front of the animal, and consequently of the shell, these 

 difficulties will be found to vanish, and a satisfactory arrange- 

 ment of nomenclature will be readily formed. 



In the true bivalves, whiph are composed of two non-symmetri- 



* Which reason induces me to refer Patella Cochlea to the Fissurclladx. 



