OSTREID^. 341 



i^ami///— OSTREID^, Lamarck. 

 [Omitted from Vol. I, p. 223.] 



The great difficulties in the systematic study of the Ostreidaj have been 

 felt by nearly all writers,^ and are due mainly to the extraordinary variation 

 in. the form of the shell. This variation has been brought about by changes in the 

 physical conditions of habitat, and particularly by differences in the character of the 

 surface to which the left valve is fixed ; it is found that the mode of growth and 

 ultimate shape of the shell are determined mainly by the size, shape, and position 

 of the attached surface, so that commonly any one species shows an amazing variety 

 of forms which can, however, be linked together by large series of specimens. A 

 further difficulty in the systematic study of oysters is due to the fact that the shell 

 usually possesses little or no ornamentation. 



Various generic or sub-generic divisions have been proposed for the Ostreidse, 

 e. g. Ostrea, Lopha ( = Aledri/onia), Arctostrea, Exogyra, Amphidonta, Grijphxa, 

 Pycnodonta, Gryphseostrea ; but it appears to me that the phylogenetic relationship 

 of the species of oysters is more complex than is represented by these divisions, 

 and that a natural grouping can only be established when more is known of the 

 evolution of the species. The forms with radial folds have usually been grouped 

 together under the name Alectryonia ; but there can be no doubt that such folds 

 have originated independently in more than one line of descent, and their presence 

 does not necessarily indicate close relationship. Thus, for example, Ostrea semi- 

 2)la7ia, which has been commonly referred to Alectryonia, appears to be more closely 

 allied to Ostrea vesicidaris than to any species of Alectryonia. The Cretaceous 

 forms which have been referred to Gryphsea clearly owe their grypheate character 

 to the small size of the attached surface,* and can be traced into forms having the 

 characters of Ostrea. In studying the Upper Cretaceous oysters of Tunis, Peron^ 

 has already noticed that some individuals of a species may present the characters 

 of Ostrea, whilst others are exogyriform or grypheate. In the Chalk of England 

 Ostrea semiplana is usually ostreiform, but occasionally, when the attached surface 

 is behind the umbo, the shell is at first exogyriform, but afterwards becomes 

 ostreiform. 



1 See, for example, the remarks of Hill and Vaughan, " Lower Cretaceous Gryphaeas of the Texas 

 Region" ('Bull. U.S. Geol. Survey,' No. 151, 1898), p. 24; and Perou, " Descript. Brach., etc, 

 Terr. Cret. Tunisie " (1890—91), pp. 105—109. 



- See R. T. Jackson, " Phylogeuy of the Pelecypoda. The Aviculidae and their Allies" ('Mem. 

 Boston Soc. Nat. Hist.,' vol. iv, 1890), p. 317 i] H. Douvillc, 'Bull. Soc. gt'ol. France,' ser. 4, vol. x 

 (1910), pp. 635, 642. 



' Op cit., p. 107. 



45 



