and their Geographical Distribution.’ 491 
+ 
so covered with this shell, that from twenty to one - 
hundred might be taken up on a single leaf. I have 
observed it in only a few other situations, — it 
occurs solitary. 
The distribution of the marine shells i is more wor- 
thy of notice. There is a peculiarity in the topog- 
raphy of Massachusetts, which causes as remarka- 
ble a modification in the distribution of the Mol- 
lusca as can be anywhere shown within the same 
limits. I refer to what is well called the right arm 
of our Commonwealth, Cape Cod. This’ neck of 
land, which stretches out into the sea, in a curved di- 
rection, a distance of some forty or fifty miles, and is 
nowhere more than about twenty miles wide, has 
hitherto proved an efficient barrier to the migration 
of many genera and species. Many whole genera 
do not pass from one side to the other of this limit. 
Thus, no species of Panopea, Glycymeris, Cyprina, 
Terebratula, Cemoria, Cancellaria, Rostellaria, or 
Trichotropis has yet been found to the south of the 
extreme point of Cape Cod; while Corbula, Cochlo- 
desma, Cumingia, Montacuta, 'l'ornatella, Cerithium, 
Ranella, and Pyrula do not pass to the north of it. 
Of the 203 marine species, 81 do not pass to the 
south, and 30 have not been found to the north of 
the Cape, though many of them approach within a 
very few miles of each other. The remaining 92 
species take a wider range, and are found on both 
sides. It is probable that future observations will 
reduce the number of the limited species somewhat, 
but still the numbers will be so large : as to exhibit a 
most remarkable limitation. 
$c: 
