348 



THE AMERICAN NATURALIST [Vol. XLIV 



not succeed in finding any with young spat attached. 

 The dark color of the winkle, of course, makes it easy to 

 overlook the smallest spat, and besides, these shells are 

 frequently speckled or spotted with plant colonies such 

 as Ralfsia verruscosa, the small colonies of which may 

 simulate young oyster-spat in size, shape and color. 

 One can distinguish the difference with a lens or by feel- 

 ing them with a knife-blade. The young of Crepidula 

 formicata, a low conical-shelled gastropod, is sometimes 

 more difficult to distinguish, but with a knife-blade one 

 can slide it along the base of attachment or pry it off and 

 note that there is no under shell but a broad clinging and 

 creeping foot. Anomia is one of the closest relatives of 

 the oyster, but, from its shape and color, is usually not 

 difficult to distinguish. Upon prying it off, the thin lower 

 valve of the shell can be seen to have a hole through 

 which a short stalk of attachment passes and permits 

 movement. The oyster becomes fixed by means of a se- 

 cretion, presumably from the edge of the mantle, which 

 cements the left valve close and fast to the supporting- 

 rock or shell. 



The spat caught on glass did not, of course, occur in 

 regular order of progression in size: the first measured 

 .87 X 1-03 mm. in height and length, the second 1.58 X 

 1.20, the third .51 X .55, the fourth .86 X .95. Similarly 

 the first found on an oyster shell measured 2.4 X 2.3, 

 while those subsequently procured varied from less than 

 1 mm. to 6 mm. in height. 



The shell of the larva is longer than high, and this U 

 true not only for each valve but also for the whole shell, 

 even when the far umbo, through tilting of the shell, 

 stands up above the near one. The youngest spat agree 

 also with this statement, but when about 1 mm. in height 

 the proportions become reversed, and from this time for- 

 wards the shell grows fastest below and at the postero- 

 inferior angle. On this account it is more useful at first 

 to build comparative measurements on the height rather 

 than on the length— the height, both for the larva and for 



