NOTES ON GRYPH^A INCURVA. 51 



weight of the heavy umbone, which before had been its safety, 

 became its ruin, for it was quite impossible for the animal to right 

 itself again, and in that prone position sand or mud was a con- 

 stant occupant and must have caused disease and death. Again, 

 as we have seen, the protection of the upper valve was removed 

 in proportion as the specialisation of the shell proceeded. That 

 course can only be regarded as suicidal, to a degree at any rate. 



There is much variation in the comparative breadth of differ- 

 ent shells of G. incurva, but whether this was correlated to other 

 shell variations — as that of the thickness of the upper valve — or 

 whether it was hereditary, we have no means of judging. 



The modern oyster has a great faculty for adapting its shell to 

 surrounding circumstances, but the shell of Gryphcea is rarely, if 

 ever, deformed. Had the organism been endowed with locomo- 

 tive powers, this would have been of easy explanation ; but in the 

 absence of those powers, it seems only open to suppose that it did 

 not frequent crowded situations, or, in other words, was approxi- 

 mately or in reality a deep-sea inhabitant. Its duration of life 

 was probably about the same as the present oyster. 



P.S. — It has been objected to the above that I " assume the 

 atrophy of the upper valve," a position which my critic thinks an 

 "impossibility . . in bivalves." The objection may, I think, be 

 allowed to stand without materially affecting the observations 

 which it was the intention of the paper to put upon record. 



I should like, however, to say a few words, based rather upon 

 observation than upon anatomical knowledge, of this "impossi- 

 bility." 



The operculum of Gasteropods is, I believe, still supposed by 

 good authorities to be the homologue of the upper valve of 

 Lamellibranchs, and as many Gasteropods have lost it altogether 

 it is clear that in the same number of cases it has at some time 

 commenced to suffer atrophy. If one moUusk can, therefore, 

 abort a part of its shell, there surely can be nothing impossible in 

 a similar allied organism performing the same operation. The 

 like argument may be applied in greater or less degree to very 

 many species in which the shell has more or less become rudi- 

 mentary, the naked slugs being the most pronounced examples. 



The modern oyster, under a highly cultivated form, seems to 



