50 NOTES ON GRYPHiEA INCURVA. 



species large-shelled varieties along with those in which the growth 

 of the shell is much checked. 



Gfyphcea, unlike the present oyster, passed its life in an unat- 

 tached position, stability in that form being procurable in a differ- 

 ent manner. The tendency of the growth of the great ventral 

 lobe and umbone was to turn over the shell on to its side. This, 

 however, was prevented by the growth of the dorsal lobe, so much 

 so that the shell was kept constantly stable at an angle of about 

 45"^, and would allow of rather a large angle in which. to rock 

 should it be required. The advantages of this position, in which 

 the animal was well removed from the mud or sand, is very 

 obvious. 



Unlike the present oyster, also, in the matter of foreign 

 growths, Gryphcza incurva was a pattern. Nothing appears to 

 have been allowed to attach itself to the shell. In upwards of 

 fifty specimens examined, I could only trace the work of even a 

 sponge in three cases, and in all of these the marks had been 

 quickly healed over. This leads us to suppose that the animal 

 was possessed of an epidermal pellicle, chemically adjusted to 

 prevent the adhesion of other organisms. 



This mollusk, though variable within certain limits, remained 

 practically unchanged for a very long period of time (Lias to 

 Chalk at least), and how it became finally extinct we do not know. 

 Like the oyster of to-day, it probably had many enemies, but 

 whether it succumbed to any of them we do not know. If, in the 

 great space of time following the Chalk, which to us is a hiatus in 

 organic life, the upper valve became really rudimentary, it would 

 seem by that change to have placed itself to a distinct disadvan- 

 tage. But when we remember that the close-fitting valves of the 

 present oyster are but little protection against its most mortal 

 enemies, we can feel no assurance in pursuing that Hne further. 



So far as we are able to judge, the changes which took place 

 in the evolution of G. incurva are not to be expressed in terms of 

 usefulness to the animal. The ingenious means by which stability 

 was attained without resulting to attachment was satisfactory so 

 long as nothing occurred to violently disturb its equilibrium ; but 

 in the event of an accident occurring sufficient to throw the shell 

 upon its other side, the consequences were disastrous. Then the 



