558 



SHELL. 



in the direction of their length (fig. 409.); and shells, that the decay of the animal membrane 

 it is then seen that whilst many of them pass leaves the contained prisms without any con- 



Fig. 407. 



Fig. 409. 



Vertical section of prismatic cellular structure, from 



Section of the shell of Pinna parallel to the surface, external layer of shell of Unio occidens. Magnified 



showing prismatic cellular structure, cut trans- 40 diameters. 



verselu. Magnified 185 diameters. j- i i i 



necting medium; and being then quite iso- 



continuously from one surface of the layer to lated, they can be easily detached from one 



the other, some terminate in points midway. 



Fig. 408. 



saw 



Lamina of decalcified membrane of prismatic cellular 

 structure, from shell of Pinna. Magnified 185 

 diameters. 



Hence it happens that the number of the re- 

 ticulations is smaller on the interior than on 

 the exterior of the layer ; their size, on the 

 contrary, being greater. The prisms are seen 

 to be marked by delicate transverse striae, 

 closely resembling those observable on the 

 prisms of the enamel of teeth, to which this 

 kind of shell-structure may be considered as 

 bearing a very close resemblance, except as 

 regards the mineralising ingredient. If a si- 

 milar section be decalcified by dilute acid, 

 the membranous residuum will exhibit the 

 walls of the prismatic cells viewed longitudi- 

 nally ; and these will be seen to be more 

 or less regularly marked by the transverse 

 striae just alluded to. It sometimes happens 

 in recent, but still more commonly in fossil 



Vertical section of cellular structure of Pinna ; at its 

 lower part the membrane is splitting into thin layers. 

 Magnified 74 diameters. 



another without any fracture. A group of 

 three such prisms, found in a fragment of 

 chalk, is shown m fig. 411.: it is seen that 

 these also exhibit transverse striae of a si- 

 milar aspect. By submitting the edges of 

 the membranous walls of the prismatic cells 

 divided longitudinally (as \nfig. 410.) to a high 

 magnifying power, the cause of the transverse 



