STRUCTURE OF SHELLS. 395 



form ; for even where almost the entire thickness is made 

 up of the prismatic substance, as in Pinna and its allies, 

 there is still a thin lining of nacre, which I shall presently 

 show to be but a simple modification of the ordinary mem- 

 branous structure." 



Of this membranous structure there are two kinds, the 

 nacreous and the tubular. The nacreous or pearly is pro- 

 duced by the membranes being crimpled with numerous 

 very delicate folds ; and these most fine folds being repeated 

 in a regular manner, give to the pleased eye the mother-of- 

 pearl lustre which so brilliantly distinguishes the inner 

 surface of many shells. The nacre or pearl is not then 

 produced, as has been maintained, by the alternation of 

 numerous layers of membrane and calcareous matter ; but is 

 clue " to the plication or folding of a single layer, in such a 

 mode that the folds shall lie over one another in an imbri- 

 cated manner." 



The other kind of membranous structure is the tubular. 

 " All the different forms of membranous shell-structure are 

 occasionally traversed by tubes, which seem to commence 

 from the inner surface of the shell, and to be distributed in 

 its several layers. These tubes vary in size from about the 

 1 -20,000th to the 1 -2000th of an inch; but their general 

 diameter, in the shells in which they most abound, is about 

 1 -4500th of an inch. The direction and distribution of these 

 tubes are extremely various in different shells ; in general, 

 where they exist in considerable numbers, they form a net- 

 work, which spreads itself out in each layer, nearly parallel 

 to its surface ; so that a large part of it comes into focus at 

 the same time, in a section which passes in the plane of the 

 lamina. From this network some branches proceed towards 

 the nearer side of the section, as if to join the network 

 of another layer; whilst others dip downwards, as if for a 

 similar purpose. The most characteristic examples of this 

 structure which I have met with are to be found in the 

 outer yellow layer of Anomia ephippium, the external layer 

 of Lima scabra, and in Chama florida. In other instances, 

 the tubes run at a distance from each other obliquely 

 through the shelly layers, and they are then usually of large 

 size. This is the case, for instance, in Area noae and Pec- 

 tunculus. In no cases have I seen any such variation in the 

 size of the tubes of the same shell, as would convey the 

 idea of their resemblance to bloodvessels ; and even where a 

 division occurs, the size of each of the branches is usually 

 equal to that of the single trunks. Sometimes these canals 

 are quite straight, whilst in other instances they arc sinuous. 



