48 LAND AND FRESH-WATER SHELLS. 



placed obliquely to the surface of the shell ; the nacreous layer 

 consists of calcareous laminae alternating with organic laminae. 

 The last is formed by the cells of the general mantle surface, the 

 epicuticula by cells near the mantle-edge, and the prismatic layer 

 by cells posterior to those forming the epicuticula. Miiller 

 believes the shell to be alive, growing by intussusception, or the 

 deposition of particle between particle ; while Tullberg seems to 

 hold the view that the organic matter is produced by fibrillation 

 of the cells of the mantle. The calcareous portion of the shell 

 consists in the main of calcium carbonate, with, sometimes, traces 

 of calcium phosphate, alumina and silica. About one-third of the 

 animals of Anodonta cygnea which I have examined were the 

 subjects of what I may term the pearly di^Jiesis. Pearls are 

 formed by depositions of nacre round a grain of sand which has 

 gained admission into the pallial cavity, provided, I would say, 

 that the organism of the animal be predisposed, since sand- 

 grains, by the very nature of their habitat, must gain admission 

 into the mantle cavity of all mussels indiscriminately, while it is 

 only in about a third that the grains of sand become surrounded 

 with nacre. It is a point well worthy of mention that when pearls 

 occur they are generally situated in and around the pericardium. 



The shell of Spkcenttm corneum has no prismatic layer, and the 

 nacreous layer has a reticulated structure. The shell is canilicu- 

 lated, and into each canal a process of the mantle extends. The 

 structure of the shells of our other Lamellibranchs are worthy of 

 investigation, and I would recommend it as a study to those of 

 my readers more especially interested in the histology of the 

 Mollusca. 



Examine the animal within its shell. Note : 



1. The mantle or pallium divided into two lobes, a right and a 

 left. Its thickened ventral border, th e pallial muscle. 



2. The ///.$ or ctenidia, lying internally to the mantle, two on 

 each side of the body. 



3. The labial palps, two pairs of triangular folds in front of 

 the gills. 



4. The/<?0/ and visceral mass, a large plougl share-shaped mass 

 lying between the right and the left mantle-lobes. 



5. The anterior and posterior adductor muscles passing from 



