PHYLOGENY OF THE PELECYPOUA. 321 



Til adult Ostrea mrginiana, the It'll valve is concave, and the right flat, when grow- 

 ing naturally, the left valve heing lowermost; and Kyder notes that the same relative 

 shape is held hy oysters which grow vertically in croAvded heds. Oysters frequently 

 attach themselves to the lower side of objects, so that the left valve is uppermost, and 

 right valve lowermost, the reverse of the common position, PI. xxv, fig. 8, and here 

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

 li^ht flat. The shell of the oyster is so variable that we should expect it to be especially 

 susceptible to varying conditions of environment; yet these strikingly different positions 

 ]iroduce no change in the resulting form. Whatever the cause of the typical form of 

 the group, the changed relations brought about by inversion, or other positions assumed 

 by the growing valves, are insufhcient to modify greatly the form of a modern individ- 

 ual, where hereditary or othei- influences bind the sheU within the limits of its typical 

 form. 



Professor Hyatt (29) has published the view that the attached supported valve of fixed 

 bivalve shells is the larger, the element of gravity being the active factor according to 

 his ideas, and he ascribes the form of the oyster and similar shells to such conditions of 

 environment. My conclusions on the form of attached Pelecypod shells, although some- 

 what similai', differ from those of Professor Hyatt. The conclusions, as presented in 

 these pages, are that the form of fixed Pelecypoda is due to the conditon of cemented fix- 

 ation; whei-eas his assume that the form of inequivalvular bivalves is due to sujjjjort, 

 the shell being free or not as the case may be. The two views are therefore based on 

 different grounds of consideration, which should be borne in mind in a comparison of 

 our studies. 



The family of the Ostreadaa is characterized throughout by being attached, at least 

 when young, by the left valve, as shown in PI. xxv, fig. 8, and PI. xxvi, fig. 13. The 

 left attached valve is tjqjically, deeply concave, of rough, irregular growth and com- 

 paratively thick. The right free valve, on the other hand, is flat or flattish, thinner, 

 and less irregular. To pei'ceive the truth of this statement it is necessary to look over 

 a collection of the Ostreadse, or in default of it some of the illustrated monographs, as 

 Coquand's or White's. It will then be seen that all typical foi-ms are referable to this 

 description in a more or less marked degree. I say more or less marked, because some 

 species, especially among the Alectryonia division, have i-ight (free) valves which are 

 comparatively like the left valves; but on careful study one finds that it is only a matter 

 of degree and that they are actually less concave and generally less irregular than are 

 the left attached valves. 



The systematic position of the family to which the oyster belongs has long been a per- 

 plexing problem to the naturalist; the asymmetry and irregularity of the valves has 

 rendered them a group dithcult to characterize and of uncertain systematic position. 

 The late Mr. Tryon in his work on Conchology says: "The union of the OstreidiB 



the attached valve of Ilippiirites. Citlceola saitdriHyin, an cratiou sti-ougly recallin,!; tlie similar structure iuduced in 

 operculatcd coral, has a deep thick attached valve and Pelecypods hy the condition of cemented lixation. Wood- 

 flattish thinner free valve; bearing thus considerable ward (77) suggested that septa and other forms of cam- 

 analogy to the efl'ects produced in bivalve molluscs by eration in Molluscs are induced by the mechanical necessity 

 cementation. Beatricea, a fossil of uncertain allinities, of cutting ott' the rear unoccupied portions of the chamber 

 has well-marked camerated structure. In all these at- of a shell as the animal is urged forward by the growth 

 tacheil forms there is an Irregularity of growth and cam- taking place on the free edges of the living chamber. 



