490 . Shells of Massachusetts, 
v 
^ 'jng a more and more practical aspect. It is pursued 
with a view to ascertain the value which fossil con- 
chology may have in determining the age and nature 
of geological formations. Various causes are found 
to modify. the distribution of shells, such as latitude, 
altitude, inelination of the surface, the character of 
the rocks, &e. On limestone formations snails are 
usually abundant, while they are rarely found in 
granite regions. T'he Limnza is most abundant in 
the streams of colder regions, and Melania takes its 
place in warmer climates. T 
The geological structure of this State, and its lim- 
ited territory, furnish little room for variety. from , 
these causes; yet a few peculiarities in distribution 
may be noticed even here. Helix hortensis is as-yet 
confined to some limited parts of the sea-coast, as the 
extremities of Cape Ann and Cape Cod ; while 
H. tridentata, hirsuta, and monodon, are only found 
in the interior and western portions of the State 
Of the fresh water mussels, we find Unio complana- 
tus, radiatus, and probably nasutus, in every region. 
U. cariosus is only found in the Connecticut and its 
tributaries, and in ponds in Plymouth. Anodon im- 
plicata is perhaps -entirely limited, in this State, to 
ponds in Essex and Middlesex counties; and A. un- 
dulata to the Blackstone and its tributaries. The 
region of the sea is said to favor the developement 
of some species of land shells; and special mentio 
is made of Bulimus lubricus. If I may judge from 
a single observation, this remark will hold good with 
us. I have seen the fallen leaves all over Oak Island, 
a small wooded spot in the vicinity of Chelsea Beach, 
