494 Shells of Massachusetts. 
about four or five inches below the surface. This 
may demonstrate to us what has happened in the 
fossil rocks of other ages, and what' will be present 
to the geologist who may live when the present soils 
become condensed into solid rocks. | 
Experience has shown, that, in order to construct a 
tolerably complete catalogue of the shells of any re- 
gion, it is necessary that observations should be ex- . 
tended through a series of years. It has been found, 
that species, which, in one season, appear in great 
abundance, are not seen again for several years, Or 
but very sparingly. In the spring of 1838, Osteo- 
desma hyalina was strewed upon Chelsea Beach 
in great numbers, and of very large size. It had 
never been observed there before, and has scatcely 
been seen there since. Cyprina Islandica, Sole — 
mya velum, Venus gemma, and Margarita arctica 
also present instances of similar periodicity at long 
intervals. In the summer of 1840, large numbers of 
` Janthina fragilis were found upon the shores of 
Nantucket; and this is the only occasion on which 
I have known of its appearance upon. our coast. 
During the winter of 1838 — 9, that remarkable shell, 
Nucula thraciaformis, was frequently found in the 
stomachs of the sand-dab (Pleuronectes dentata) ; 
but the search for them since has been almost fruitless. 
These fish were taken off Race Point, the extremity 
of Cape Cod, and this is the only locality where the 
shell has yet been found. Many similar instances 
might be mentioned. Hence, in addition to the te 
mark made above, it may also be suggested, that It 
is always best to lay in a good store of any species, 
"^ While we may. Tie 
