JONES: MOLLUSCAN ALBINISM. 9 
to speak more correctly, a semi-transparent shell, may possess 
no colour, yet not be white, and still an albino. The fact is 
that a shell appears white when it is opaque, because all the 
rays are reflected from it; it does not appear white when trans- 
parent, because it allows the rays of light to pass through. 
These two modifications of albinism can be well studied on a 
specimen of “4. virgata var. hyalozonata, where they may be 
seen side by side. Such a thing as a really transparent shell 
does not exist, but a good test for a semi-transparent one is to 
lay the specimen ona sheet of white paper—if any colour is 
seen it is not an albino. 
The title of this paper contains the word ‘Tendency.’ By 
“tendency,” I mean to indicate a general and wide-spread in- 
clination amongst shells last year to occur lighter in colour than 
normally. The word ‘tendency’ may also, and more correctly, 
perhaps, be used to indicate the frequency of albinism proper dur- 
ing last year. Whether this observed frequency is in reality due 
to an unusual prevalence of the phenomenon last year, or whether 
it is an outcome of investigation into the subject, I cannot say. 
In many cases where the albino variety of shell is found the 
typical specimens are extremely light in colour. At Birch, for 
instance, the typical shells are of a light straw or amber colour, 
which is quite different to the usual condition. Ayalinia 
cellaria at Disley in Cheshire, and at Matlock, were last year of 
a very faint yellow colour; A. cel/arta, however, is a shell 
normally light in colour. Z. g/abra is a shell usually dark, but 
last year at Disley, and this at Marple, specimens were as light 
as Hf. cellaria. ‘The locality at Marple is that in which this shell 
was discovered by Mr. Thomas Rogers, and has naturally been 
under surveillance for a long time, and it is noteworthy that a 
light form of this species has never been met with there before. 
The lightness may be due to one of two causes—the usual 
amount of pigment granules may be deposited of a lighter 
colour, or fewer granules of the normal shade may be produced. 
The fact that these light coloured individuals have been taken 
