508 MOLLUSCOUS ANIMALS GENERALLY. 



happens in recent, but still more commonly in fossil shells, that 

 the decay of the anynal membrane leaves the contained prisms 

 without any connecting medium; as they are then quite isolated, 

 they can be readily detached one from another; and each one 

 may be observed to be marked by the like striations, which, 

 when a sufficiently high magnifying power is used, are seen to 



be minute grooves, ap- 



FIQ. 25i. parently resulting from 



a thickening of the in- 

 termediate wall in 

 those situations. This 

 thickening seems best 

 accounted for, by sup- 

 posing (as first sug- 

 gested by Prof. Owe*n) 

 that each long prisma- 

 tic cell is made up by 

 the coalescence of a 

 pile of flat epidermic 



Oblique Section of Prismatic ShelJ-substance. Cells, the trail SVCl'Se 



striation marking their 



lines of junction ; and this view corresponds well with the fact, 

 that the shell-membrane not unfrequently shows a tendency to 

 split into thin laminae along the lines of striation ; whilst we 

 occasionally meet with an excessively thin natural lamina, com- 

 posed of flat pavement-like cells, lying between the thicker 

 prismatic layers, with one of which it would have probably coa- 

 lesced, but for some accidental cause which preserved its distinct- 

 ness. That the prism is not formed in its entire length at once, 

 but that it is progressively lengthened and consolidated at its 

 lower extremity, would appear also from the fact, that where the 

 shell presents a deep color (as in Pinna nigrina\ this color is 

 usually disposed in distinct strata, the outer portion of each layer 

 being the part most deeply tinged, whilst the inner extremities 

 of the prisms are almost colorless. This prismatic arrangement 

 of the carbonate of lime in the shells of Pinna and its allies, has 

 been long familiar to conchologists ; but it has been usually re- 

 garded as the result of crystallization. 



337. It is only in the shells of a few families of Bivalves, that 

 the combination of organic with mineral components is seen in 

 this very distinct form ; and these families are for the most part 

 nearly allied to Pinna. In all the genera of the Margaritacece, 

 we find the external layer of the shell formed upon this plan, and 

 of considerable thickness ; the internal layer being nacreous. In 

 the Unionidce (fresh-water Mussels), on the contrary, nearly the 

 whole thickness of the shell is made up of the internal or nacre- 

 ous layer ; but a uniform stratum of prismatic cellular substance 

 is always found between the nacre and the periostracum. 1 In 



1 The periostracum is the yellowish-brown membrane covering the surface of many 

 shells, which is often (but erroneously) termed the epidermis. 



