ANATOMY AND LIFE HISTORY OP MOLLUSCA. 155 



inside, and the other at the periphery. The inside lining is 

 composed of the thinnest layers of shell, and partakes of the colour 

 of the nerve-tissue, which, as we shall see by and by differs very 

 much in different genera and species. But roughly speaking, as 

 I have already stated there is a division into certain plates, three 

 or four in number. I have given a description already of one such 

 section from outwards to within. At the risk of being tedious I 

 will repeat what I have observed in Patella tramoserica, Martyn, 

 in thin sections under the microscope, proceeding from within 

 outwards. First there is the thin layer of fibrous tissue already 

 described as being full of pores for the entry or exit of the nerves. 

 The calcareous structure is faintly visible amidst the membranous 

 sheaths, in the form of faint striae inclined at an angle of 45°, in 

 two opposite directions. Then succeeds the nacreous lining in 

 which very fine nerve-channels can be seen, having an irregular 

 course, with branches at intervals. Some of these can be followed 

 a considerable distance into the substance of the shell, and they 

 are conspicuous for their irregular course and their alternate 

 dilatation and narrowing. There are fine lines of membrane lying 

 parallel with one another, which are occasionally darkened by 

 granular substance, black and brown, appearing more like foreign 

 matter enclosed by the growth of the shell than anything else. 

 The lines of growth which at their edges have lines of very fine 

 parallel rods not wider than the width of the series of cells which 

 seems to compose them are distinctly visible. Finally there is the 

 outer layer or fibrous plate containing the micropores and the eyes 

 already referred to. Probably outside this there is in nearly all 

 shells an outer layer of membrane or a periostraca ; but it has 

 generally disappeared in shells that have been out of the water 

 any length of time. 



The inner or nacreous lining though apparently opaque, with a 

 strong golden or silvery lustre, is in reality transparent • and the 

 golden or silvery colour is due to fine lines of membrane which 

 pass down the interior lining of the shell. Besides this there is a 

 thin inner plate next to the mantle which is full of transverse 

 tubes. They appear to me, in sections, not to differ in structure 

 from the fibres of the upper layer, though the result is very 

 different. They are a series of somewhat narrower and more 

 regular tubes and fibres of membrane, passing at an inclination at 

 right angles to the fibres of the upper plate, from one side of the 

 lining to the other. They are dotted at intervals with lines of 

 darker material, which are sometimes a series of small rounded cells. 



The lines of membrane seen with the naked eye in the interior 

 of the shell correspond with the strike of the nacreous plates on 

 the periphery of the shell. The fibres are sometimes separated by 

 intervals or spaces. They are probably the folds spoken of by 



