NOTES ON GRYPH^A INCURVA. 49 



G. incurva, as to how and when the departure from a primitive 

 form took place, or any hint as to the nature of that form. The 

 smallest shells procurable, which are probably those of the first 

 year, show all the characteristics equally of the adult form. We 

 can, therefore, only say that, although its first appearance is in the 

 Lias formation, its development took place probably long anterior 

 to the laying down of that deposit. 



The peculiarity in the development of the lower valve is very 

 interesting. The adductor muscle was always attached to one 

 side of the shell, technically known as the dorsal side. This leads 

 us to suppose that side to have been the primitive one, and the 

 other side, or lobe, which is marked off by a furrow on the exte- 

 rior, to have been a subsequent development. This subsequent 

 development is that which gives to the shell its generic character- 

 istics. It forms the bulk of the lower valve or umbone, and in 

 the course of its development pushes the apex towards the dorsal 

 side. Meanwhile, the growth of that side proceeds much more 

 slowly, and always holds a small relative proportion. It is, per- 

 haps, due to this great demand that the upper valve appears to be 

 so much impoverished. This valve is of very variable proportions 

 as regards thickness. Assuming that it receives its increments in 

 due course, the quantity of shell deposit must be variable in dif- 

 ferent individuals, if not at different times in the same individual. 

 This variability, if it occurs in the lower valve, is not so easily 

 detected in the sum of its results. 



The lateral position of the adductor gives rise to a conjecture 

 as to whether that position may not have something to do with 

 checking the growth of that particular side. Being large in pro- 

 portion to the surface of that lobe, and in its course traversing 

 about three-fourths of the length of the shell, it must apparently 

 always be detrimental to its growth. In this case the unsymmetri- 

 cal development of the shell may be due to the same cause that 

 shifted the adductor from its central position. This, however, has no 

 connection with the otherwise excessive development of the lower 

 valve. This seems to be bound up with the cause which has 

 given us other large developments in the oyster family in geologi- 

 cal time, and which at the present time produces in the same 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. VII. e 



