98 



EOCENE MOLLUSCA. 



orifices and the heart, all of which are on the side contrary to that in which they are 

 placed in the animals of dextral shells, induced Cuvier to consider that species as 

 sinistral : and in this opinion many eminent naturalists have concurred. M. Desmoulins, 

 however, has ascertained, by a careful anatomy of the animal of P. corneus, that, 

 although the orifices and the heart have an abnormal position, the organs of digestion 

 and generation, in fact, retain the position they hold in dextral molluscs ; and that 

 author, therefore, maintains that nearly all the known species of Planorbis, as well 

 living as fossil, are dextral. M. Deshayes concurs in this opinion ; and, after remark- 

 ing that the upper side may be distinguished from the under side by the obliquity of 

 the aperture, the superior margin of which is more produced, cites the observations of 

 M. Desmoulins as explaining the apparent anomaly of a sinistral animal in a dextral 

 shell, and how, in reality, the animal is dextral as well as the shell ; there not being 

 any other derangement in the relation of its organs than with regard to the heart and 

 the termination of the digestive and generative organs. With regard to the shell, it 

 will be seen at once, on observing the manner in which it is carried by the animal, 

 that it is dextral, and that, as Mr. Benson has stated,* if it be viewed practically as 

 sinistral, and placed as such, the animal will be on its back, and will have to twist its 

 body half round in order to gain the ground with its foot. Mr. Benson, therefore, 

 proposes to consider that face as containing the apex, in discoidal shells, which is con- 

 tiguous to the back of the animal ; and, he adds, this side may invariably be known in 

 Planorbis by the greater projection of the lip in that part, by the deeper depression 

 of the central umbilicus, and by the more considerable involutions of the whorls 

 occasioning a greater depth of suture. Mr. G. Sowerby, on the other hand, assertsf 

 that the shell is sinistral, and that it is only needful to observe on which side of the 

 shell the very apex of the spire is actually to be seen, and, taking that side for the 

 upper, in conformity with the strict rules of analogy, it will immediately be evident 

 that the aperture is on the left side. This criterion, however, is seldom available ; 

 for, in general, the apex is concealed by the involution of the w r horls, and the shell 

 presents, on each face, what may be easily mistaken for an umbilicus ; and the diffi- 

 culty in determining which face contains the apex, and which the true umbilicus, is 

 not removed. It is well known that testaceous molluscs, when placed in conditions 

 unfavorable to healthy development, frequently depart from their normal form ; and 

 that their shells, in consequence, are distorted, and become what are usually called 

 monstrosities. Thus, in Planorbis, the animal, under such conditions, frequently loses, 

 to a greater or less degree, its strong convolution on a horizontal plane ; and the shell 

 assumes, in conformity, a more or less elongated spiral form, with an elevated apex. 

 M. Desmoulins cites these monstrosities in support of his opinion, observing that, 

 when they occur, the whorls gradually glide from left to right, down the imaginary 



* Journal Asiat. Soc. Bengal, vol. v, p. 744. 

 •f Genera of Shells. Gen. Planorbis. 



