254 



STRUCTURE OF THE SHELL 



disposition of a single membranous layer in folds or plaits, which 

 lie more or less oljliquely to the general surface, so that their 

 edges show as lines. The nacreous type of shell occurs largely 

 among those Mollusca which, from other details in their organisa- 

 tion, are known to represent very ancient forms (e.g. Nucula, 

 Avicula, Trigonia, Nautihis). It is also the least permanent, and 

 thus in some strata we find that only casts of the nacreous shells 

 remain, while those of different constitution are preserved entire. 

 Porcellanous shells (of which the great majority of Gastero- 

 poda are instances) usually consist of three layers, each of which 

 is composed of a number of adjacent plates, like cards on edge. 

 The inclination of the plates in the different layers varies, but 

 that of the plates in the inner and outer layer is frequently the 



Al- 



Fig. 162. — A, Seetiou of slit41 of Unio ; a, periostracal layei- ; b, prismatic ]ayer ; c, 

 nacreous layer. B, Horizontal section of shell of Pinna, showing the hexagonal 

 prisms. 



same, thus if tlie plates are transverse in the middle stratum, 



they are longitudinal in the inner and outer strata, and, if 



longitudinal in the middle, they are transverse in the other two. 



N"ot uncommonly (Fig. 163 B) other layers occur. In l)i valves 



the disposition and nature of the layers is much more varied. 



In Unio the periostracal or uppermost layer is very thin ; 



beneath this is a prismatic layer of no great depth, while the 



whole remainder of the shell is nacreous (Fig. 162 A). Many 



l)i valves show traces of tubular structure, while in the Veneridae 



the formation and character of the layers approaches closely to 



that of the Gasteropoda. Further details may be gathered from 



Carpenter's researches.^ 



^ Dr. W. B. Carpenter, Rep. Brit. Ass. xiii. p. 71 ; xiv. p. 1 ; xvii. ]>. 93 ; J. S. 

 IJowerbank, Trans. Micr. Hoc. i. p. 123 ; Ehrenbaum, Zeif. iriss. Zool. xli. \\ 1- 



