ZOOLOGY. 615 



penetrated about a millimeter into the thickness of the columellar muscle 

 and which occupies the whole length of the foot. " The walls of this pedal 

 fissure are lined with a peculiar epithehum, folded, or rather goffered, 

 so as to somewhat resemble the polypary of a Maeandrina." Then it has 

 a v<?ry delicate and very flexible transparent lamella, which can be taken 

 out with a fine needle, and which after remaining some time in the airdries 

 and acquires a horny appearance. It is found that " the epithelial cells 

 of the fissure excrete a structureless material of a yellowish color and 

 strongly refractive, which, by agglomeration, constitutes the hyaline 

 lamella. The latter issues from the pedal fissure and adds itself to the 

 old operculum." 



" The newly-formed parts apply themselves to the epithelium situated 

 between the fissure and the parts of the operculum which are already 

 fixed. At this point the epithelial cells are but feebly adherent to each 

 other, and only by their basal part, a remarkable exception among epi- 

 thelia. From this it results that the still plastic opercular material in- 

 vests these cells, and even difluses itself somewhat among the elements 

 of the subjacent tissues (muscular fibers and connective cells). It is 

 thus that by successive appositions of new parts issuing from the pedal 

 fissure the striae are formed, of which only the most salient are visible to 

 the naked eye or the lens." 



"On the other surface of the operculum we must distinguish two re- 

 gions, the surface of insertion of the columellar muscle, and the free 

 internal surface. It is very easy to see that the free internal surface 

 and the outer surface of the operculum have not the same constitution. 

 The inner surface is covered with a homogeneous layer which forms, as 

 it were, a varnish without any striae perceptible to the eye. This coat- 

 ing may even be so thick that we cannot see through it the striae of the 

 other surface. In other cases it is delicate enough not to hide them. 

 This difference of constitution arises from a difference of origin. In 

 front of the surface of insertion the metapodium forms an interior ex- 

 pansion or lip, which, during the life of the animal, is constantly applied 

 against the inner unattached part of the operculum. The epithelial 

 cells of this anterior lip produce the varnish. 



"It still remains to ascertain why the opercular material is rolled into 

 a spiral. To elucidate this point we must study the muscular impression. 

 As the operculum grows, the surface of insertion of the operculum is dis- 

 placed with a slight movement of rotation, since during the same time the 

 shell grows in a spiral. The muscle attaches itself to the newly formed 

 parts, abandoning the old parts on the side of the parietal margin. These 

 stages of the columellar muscle are marked by striae independent of 

 those of the superior surface. We observe them when preparing an 

 operculum after removing all traces of muscle. By studying these lines 

 we can even understand why there are opercula of which the form 

 always remains the same, while there are others of which the form 

 varies with the age of the animal. The latter are said to have a nucleus 



