Jackson.] 548 [April 4, 



relative shape of the valves is maintained, the left valve concave, 

 the right flat. The oyster is such a variable form that this fact 

 has a significance. We should expect it to be specially susceptible 

 to varying conditions of environment ; yet here strikingly different 

 positions find no change in the resulting form. This is evidence 

 in favor of Professor Hyatt's theory that the attached, supported 

 valve of fixed bivalve shells is the larger (11) but contrary to his 

 theory of gravitation (11), according to which the lowermost valve 

 should be thick and concave which is seen not to be the case in re- 

 versed specimens of oysters. Whatever may have been the cause 

 of the typical form of the group, gravity is insufficient to modify 

 greatly the form of a modern individual. 



Note. — Since the corrected proof of this paper went to press my attention has been 

 called to a paper by Dr. Benjamin Sharp, Remarks on the Phytogeny of the Lamelli- 

 branchs, Proc. of the Acad, of Nat. Sci., Phila., March, 1888, pp. 121-124. In it he pre- 

 sents a mechanical theory for the loss of the anterior and retention of the posterior 

 adductor muscle in Lamellibranchs, which is quite similar to that presented here in 

 the revolution of the axes. 



LIST OF PAPERS QUOTED AND REFERRED TO IN THE TEXT. 



1. H. and A. Adams. Genera of Mollusca. London, 1858, Vol. n. 



2. W. K. Brooks. Embryology of the Fresh Water Mussels. Proc. 



Am. Assoc. Adv. Sc, 1875. 



3. W.K.Brooks. The Affinities of the Mollusca and Molluscoidea. Proc. 



Bost. Soc. Nat. Hist., Vol. xvm, 1876. 



4. W. K. Brooks. Development of the Oyster. Johns Hopkins Univ. 



Biol. Lab. Studies, i, 1879-80. Reprinted from the Rep. of the 

 Commissioners of Fisheries of Maryland, 1880. 



5. B. Hatschek. Ueber Entwickelungsgeschichte der Teredo. Arbeiten 



aus dem Zool. Inst., Wien, Vol. in, 1881. 



6. R. Horst. Contribution to our Knowledge of the Development of the 



Oyster, O. edulis. Bull. U. S. Fish Com., Vol. n, 1882. A transla- 

 tion from the Dutch by J. A. Ryder. 



7. R. Horst. The Development of the Oyster. Rep. U. S. Fish Com. 



1884. Washington, 1886. Translated from the Dutch by Herman 

 Jacobson. 



8. T. H. Huxley. Oysters and the Oyster Question. English Illustrated 



Magazine, Oct. and Nov., 1883. Partly reproduced by J. A. Ryder 

 in his paper No. 23. 



9. T. H. Huxley. Anatomy of Invertebrated Animals. 



10. Alpheus Hyatt. Guides for Science Teaching. The Oyster, Clam 



and other Common Mollusks. Boston, 1880. 



11. Alpheus Hyatt. Transformations of Planorbis at Steinheim, with 



Remarks on the Effects of Gravity upon the Forms of Shells and An- 

 imals. Proc. Am. Assoc. Adv. Sc, Vol. xxix, 1880. 



