48 NOTES ON gryphj5:a incurva. 



Roman builder, mollusks appear to do their masonry by small 

 layers, allowing one to harden before the other is laid on. The 

 secretions seem to accumulate and then to be used up in shell- 

 making all at once. Gryphcea, like the oyster of to-day, was 

 great in this matter of shell accumulation — perhaps too great, as. 

 it may appear. 



A peculiarity is manifest in Gryphcea incurva^ which, so far as 

 I know, has no match in other members of the oyster family. 

 This is the circumstance that the adult animal probably could not 

 be contained in the shell when the valves were closed. This 

 would appear, however, only to apply to the adult form, in which 

 the lower valve, being much thickened on the inside, becomes 

 very shallow, for in immature form the shell is much deeper in 

 proportion and allows plenty of room. In my specimen, which 

 is perfectly adult, the lower valve is slightly recurved at the lip, 

 which I think clearly proves that the animal at that stage some- 

 what overhung its shell. This, however, does not entitle us to say 

 that the upper valve had become rudimentary, although the ten- 

 dency was certainly in that direction, for, as we have seen, the 

 upper valve formed a real protection during a great part of the 

 animal's life. Moreover, the adductor muscle, being well deve- 

 loped, shews that it w^as in frequent use. 



In considering to what part of the line of ostreal development 

 this organism should be referred, the two valves of the shell 

 would seem to furnish an answer. The elaboration and tremen- 

 dous development of the lower valve shows that the shell-bearing 

 functions were at or about at the maximum, and were nowise in a 

 nascent condition. The smallness of the upper valve is probably 

 to be explained by the same circumstance that gave such an 

 unequal impetus to the growth of part of the lower valve, to be 

 presently noted, and at the same time gave such an undue pre- 

 ponderance of shelly matter to that section. 



These considerations, coupled with the tendency to abort an 

 important organ (see above paragraph), seem to stamp the form as 

 specialized and therefore later, and not ancestral or earlier. This 

 view will receive further support in considering the development 

 of the lower valve. 



There is no clue given by the study of the immature shell of 



