452 NOTES. 



Neritina, otherwise the two groups would long since have been united 

 in one genus. But the differences in their structure, irrespective of the 

 form of the shell and the structure and situation of the operculum, are 

 so trifling that we are justified in regarding the Navicella as a modified 

 form of Neritina. 



Note 100, page 212. It is in accordance with this view that we find 

 great variability in the forms of the operculum of Navicella. We know 

 that truly rudimentary organs which have lost their principal function 

 and have not become serviceable for any other well-defined function are, 

 as a rule, remarkably variable. That is the case here. While the oper- 

 cula of the most dissimilar Neritinas are of very uniform structure, in 

 those of Navicella we find the widest dissimilarity. My own researches 

 which certainly are not yet completed make it seem probable that the 

 deviations from the normal form in a species, even in individuals, may 

 sometimes be very considerable. It is, moreover, a very interesting fact 

 that even the operculum of the male may differ from that of the female. 



Note 101, page 213. The structure and origin of the shells of mol- 

 lusca are not, even at the present, thoroughly understood ; for although, 

 as to the first point, a vast mass of interesting researches lies before us, no 

 one has yet succeeded by purely histological investigation, and a know- 

 ledge, however exact, of the minutest structure of the shell itself, in 

 establishing any fundamental character as common to all shells. At 

 the same time too much attention has been paid to the general relations 

 of the shell when fully developed, and not enough to the development 

 and bearings of those relations. We know, for instance, that in the fresh- 

 water mollusca three distinct and very dissimilar layers are to be found. 

 The external layer is a purely organic cuticle ; next comes the prismatic 

 layer ; and inside this the nacreous layer. It is admitted that the pris- 

 matic layer is often wanting. Now, if we are considering merely the 

 details of structure, the correctness of this view cannot be doubted, but 

 it would be quite an error to suppose that every shell in which the pris- 

 matic layer was wanting therefore consisted of two layers only, the outer 

 cuticle and the internal nacreous layer. In point of fact, these shells 

 also have three separate layers, and that lying next beneath the organic 

 cuticle differs from a true prismatic layer only by its deviation from it 

 in the physical process of formation ; the material composing it does 

 not form distinct prisms as it is secreted. But the spot whence it 

 originates is, on the other hand, an essential character. The organic 

 cuticle and the prismatic layer the external calcareous layer are se- 

 creted only and invariably at the margin of the mantle ; the former fre- 

 quently between two lobes, or folds of the margin of the mantle, the 

 second from the narrow edge between the margin of the mantle, and a 

 line, not always present, which indicates the insertion of the small 

 muscle of the mantle. From this line as far as to the top of the shell, 

 nothing can be secreted but nacreous material ; the growth in thickness 



