I30 



BULLETIN OF THE BUREAU OF FISHERIES. 



attached to the periostracum when that is peeled off. Beneath the prismatic layer and 

 composing nearly the entire body of the shell is the nacreous or mother-of-pearl layer, 



which is made upof almost 

 innumerable thin laminae 

 lying one upon the other 

 and parallel to the iimer 

 surface of the shell. 

 Through the nacre, inter- 

 secting its laminae .passes a 

 very thin fourth layer, the 

 hypostracum," secreted 

 by the muscles (p. 172). 



Growth of shell in 

 thickness is accomplish- 

 ed by the laying down of 

 successive laminae, from 

 the entire surface, of the 



Fig. I. — Diagrammatic and highly magnified camera lucida drawing of section of 

 margin of fresh-water mussel shell. Obovaria ellipsis (IvCa), showing arrangement of 

 layers: A, epidermis (double layer); B, prismatic layer; and C, nacreous layer. 

 Note the folds of epidermis which give the shell its "silky" appearance. 



mantle. Layer after layer is added to the inner surface of the shell, each layer exceed- 

 ingly thin and generally a little larger than the preceding. Ring after ring is added 

 to the margin of the shell, but since gro%vth is most 

 pronotmced in the posterior (rear) direction, less 

 so in a ventral, and still less in the anterior 

 (forward) direction the rings must be widest be- 

 hind and narrowest in front. It will be noted 

 that any mussel shell is marked with innumerable 

 concentric lines. Superficially such lines suggest 

 the annual rings seen on the section of the trunk 

 of a tree, but the resemblance is entirely mislead- 

 ing. The shell is added to in layers, but a very 

 great number of layers are made in a year. 

 Pfund (191 7) has, by refined physical methods, 

 measured the thickness of the layers or laminae 

 and determined that the thickness in the examples 

 he studied lies between 0.4 /u and 0.6 ix. Transla- 

 ting these terms into ordinary language, there are 

 some 50,000 layers to an inch of thickness. A shell 

 one-quarter of an inch thick would have 12,500 lam- 

 inae ; and if such a shell were 8 years old, more than 

 1,500 laminae would have been formed each year, 

 on the average. The outcropping edges of these 

 laminae on the surface of a polished niggerhead shell 

 have also been measured and found to be spaced at 

 the rate of about 9,000 to the inch. Such lines are of course not visible to the naked eye, 

 and therefore the fine rings in evidence on the surface of the shell can not represent these 



Fig. 2. — Section through double-layered peri- 

 ostracmn and prismatic layer. Nacreous layer 

 below not shown. 



«» Not shown in figures herewith. 



