MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 333 



owing to the difificulty of examining them aUve. In the first 

 place, the mere fact of the removal from the shell cannot be 

 effected without grave injury, and very few of the marine 

 species can be kept alive for any length of time after their 

 capture, while all preservative fluids distort them so much that 

 they cannot be satisfactorily examined unless by tedious methods 

 of dissection and sectionizing. 



Moquin-Tandon, however, describes LiinncBa miricularia 

 with two heads, and Physa acuta with bifid tentacles. My 

 remarks will therefore be restricted to the shell alone, and it is 

 to the shell that most attention has been paid. 



The systems of Conchology, at all events all the older ones, 

 are based on the characters which the shell presents. There is 

 no doubt that the form of the shell is intimately correlated with 

 the structure of the animals, although in some cases, if the form 

 of the shell be alone considered, mistakes will be made. To 

 take an example from our own fauna, Asnviinea grayana would 

 be at once placed in the family Littorinidce, but the animal 

 shows that it belongs to a totally different group. 



It is, therefore, necessary in the first place to consider what 

 is the relation of the shell to the animal itself, and how it is 

 formed. The shell is the product of a single organ, viz., the 

 mantle, and all the markings and variations of the shell are 

 produced by corresponding peculiarities of that organ. In the 

 vast majority of cases the shell is external, and is generally large 

 enough to enclose the whole of the soft parts, but in some cases, 

 as in the Accra and Bulla, part of the animal is permanently 

 external to it. 



The Nudibranchs, which are all marine, have no shell 

 whatever. In the few cases where the shell is formed in the 

 interior of the animal, as in the Li/naces, I am not aware that 

 there is much variation either in form or colour in any species, 

 so that I need not further refer to them. In this case only the 

 more important vital organs are protected by the shell. The 



