8 



posited as tl If y are internally, as growth proceeds pro- 

 ject in succession past each other at the tree edges of the 

 valves and external surface of the shell, so that the suc- 

 cessive deposits may frequently be distinguished, espe- 

 pecially. as these deposits seem to be laid doAvn periodi- 

 cally, giving rise to a very rough imbricated appearance 

 of the edges of the layers r>n the outside of the shell. 

 The inner pearly or nacreous layers of the shell are dif- 

 ferent in structure fr< jui the outer ones. These are made 

 up of microscopic prisms of carbonate of lime embedded 

 in a matrix of horny substance, analogous, if not identi- 

 cal Avith the substance which forms the external hard 

 parts of insects The outer portion of the shell, forming 

 much the largest ]3roj3ortion of the whole, is diiferently 

 constituted according to Xathusius-Konigsborn^. who 

 hnds that in the shells of the oyster of Europe there are 

 superimposed cavities, tilled in life with water, which 

 lie just beneath the inner pearly layers and between the 

 attachment of the muscle and the hinge corresponding 

 to the thickened nacreous portion. In the thick margi- 

 nal portion of the shell, extending from a point half way 

 between the insertion c>f the muscle and hinge, to near 

 the edge of the valves, he linds still another series of 

 closed cavities which are separated by ol^lique divisions 

 of a laminar character, and which are tilled with a singu- 

 lar structure < jf minute cavities which contain air. The 

 first kind or the water bearing cavities are. it may be 

 well to remark, greatly developed in ^pondylu.^. an ally 

 of the oyster. The laminar character c»f the shell is 

 apparent from the presence of very tine lines of growth 

 even on the valves of the free-swimmino' embryos, and 

 in the embryonic shell which covers like a little i-aised 

 cap the extreme tips of the umbos or hinge ends of the 

 valves of attached spat x tu 1 inch in diameter. 



During the past season I endeavored to ascertain if it 



^Untersuchungen ueber nicht ce.lul're OrgaEismen, namentiich ; Crustaceen-panzer. Mol- 

 lusken — Schalen und Eih611en.~ Von W. Von Nathusius-KCnigsbom. 4 to., pp. 14^4, pis. 16. 

 Berlin, 1877. 



