64 



tionary basis B, shows that the right valves Bs. is 

 concave within and convex without, while the attached 

 left valve Ls. is Hat conformably to the character of the 

 basis of hxation, which is most often of this nature. 

 Contrary to what is noted in regard to the shape of the 

 valves in the "spat/' the right valve of the adult Rt\ is 

 nearly Hat, while the left one Lt. is most decidedly con- 

 cave within and convex without. Other changes in the 

 more important parts during growth are manifested 

 mainly by the muscle Jfics., which moves the position of 

 the insertions more towards the free edges of the valves, 

 while the ligament Ug. is moved in the same direction. 

 Other differences in the forms of the valves at the differ- 

 ent stages may be noticed ; for instance, the sliells of the 

 "fry" are deeply concave and very thin, the valves of 

 the "spat" are still also relatively thin, while it is only 

 the adult which has a very thick, stout shell, though the 

 thickness is said to vary with the depth of the water, the 

 thinnest shelled oysters being found in the deepest water. 

 These deep-water oysters are known as "paper-shells" 

 and are not liked by oyster packers and dealers, because 

 the valves break very easily in opening tliem, causing 

 additional labor in tlieir removal. This last fact is evi- 

 dence in favor of shallow- water culture. 



There are good grounds for dividing the groAvth of the 

 oyster into the three stages already alluded to : the first 

 we may consider as answering definitely to the free 

 swimming condition or that of the "fry ;" the second the 

 thin-shelled primai'y fixed condition or that of the 

 "spat :" thf third, that of the adult, which already has 

 a pretty thick shell wlien a year old, when the generative 

 organs too are in active functional use, a circumstance 

 which has also Inn^n notice<l by writers on the life-history 

 of tlie oyster of Kuro})e. 



