38 
STRUCTURE OF SHELL IN BIVALVES. 
except the free inargiii of the valves, being secreted by the wliole 
external epithelium of the mantle, and consisting of a number of 
superimposed laminai laden with calcareous particles, as many as 
twelve distinct laminai were clearly separated in a valve of an 
apparently adult Anodoutu cnguea from Eurwell fish-pond, and we 
have possibly in this feature another indication of the age of the 
mollusk. In the genus Unto the prismatic layer is comparatively 
thin, while the harder and more compact nacreous or inner layer is 
very thick. 
Though the shells of our Bivalves are approximately ecpial in 
thickness thro\ighout their entire extent, yet at maturity there is in 
most species a noticeable submarginal thickening developed around 
the free edges of the valves, this thickening also occurs, though 
less markedly, at the termination of each seasonal or periodic stage 
of growth. ]Mo(piin-Tandon atlirms that the Bivalves are thickest 
near the umbones, but although this may probably be the case in 
most species, and even also in the Uiiloiies, when under the stimulus 
of erosive action, they have ahuormally and enormously thickened 
the umbonal region, l)y secreting additional layers of calcareous 
matter, as a protection against the destructive action of acidulous 
waters ; yet usually the thickest part in those shells is the area on 
the anterior side near the ventral margin, and in position almost 
coincident with the anterior part of the i)allial line and really forming 
part of the Ventral Crest of Picard, this thickened part is chiefly com- 
posed of the nacreous layer, the prismatic layer in the genus Unio 
having its greatest thickness, both comparatively and actually, at 
the posterior end of the shell. 
The left valve of a specimen of A nodonta cugnea collected by Mr. 
W. 1). Roebuck, F.L.S., in Burwell Fish-pond, near Louth, weighing 
27 '506 grams, was analysed by Mr. Crowther by the wet process — 
the same method as was adopted in the analysis of the shells of 
Iltdiv jxinidtia and Limnau ><Uign(dh, the results of which were given 
at p. 20 — and gave as a result 25’231 grams of inorganic, and 2'335 
grams of organic substances, or 91 '53 per cent, of inorganic earthy 
salts, and 8'47 per cent, of organic matter. 
The right valve of another adult specimen from the same locality 
weighed 36‘8G1 grams, and of this about one-third was analysed by 
the dry method, and burned to Calcium oxide, CaO, when the loss of 
moisture, organic matter, etc. was found to be 2U U08 per cent. 
