190 THREE CRUISES OF THE ' BLAKE.” 
in S. pinnata, side by side in a single row, stretching vertically 
across the whole width of the chain, and forming a thin ribbon, 
which when floating is usually slightly coiled like 
a tape. The 200145 of the chain resemble S. Afri- 
cana. This species was found from Cape Hatteras 
as far north as the eastern extremity of George’s 
Shoal. 
The sudden arrival of innumerable Salps on 
our coast is most interesting. It is not unfrequent 
for the northern species of Salpa (Figs. 110, 111), 
so common along the eastern coast from Cape 
Hatteras to Cape Cod, to make its appearance in a 
single night in such masses as to discolor the sea 
for miles near the entrance of Narragansett Bay, 
and to remain swarming for a couple of months, 
when it disappears as quickly and mysteriously as 
it came. The explanation of this sudden inroad 
is probably due to the fact, that during the time 
they are sterile the solitary individuals remain at 
some distance below the surface, but when they 
begin to bud and form the chains they come near 
the surface. It is easy to explain their abundance 
then by the rapid development of the young 
chains, which are formed, thrown off, and increase 
The pteropods and heteropods are by 
far the most common pelagic forms of the 
mollusca, the dead tests of the former be- 
ing literally heaped up in beds on the 
bottom of the sea in deep water. The 
dredge often eame up completely choked 
with pteropod shells, showing what an 
Ò important part they play in building up 
the deep-sea deposits by the decomposi- 
Fig. 111. —Selpa hes tion of their tests. In former geological 
Chain, somewhat enlarged. periods, when there were gasteropods 
allied to pteropods like Bellerophon, of gigantie size, their 
effect in forming bottom accumulations must have been still 
