ON BRITISH MARINE ZOOLOGY. 251 
though many dead, the number of dead univalves predominating in the one 
instance, and of dead bivalves in the other, respectively 17 and 20, both high 
numbers. 
Twenty-six papers from the Irish sea relate to a sufficiently limited range 
in depth to admit of a similar inquiry. Of these, eight included more than 
10 species of univalves and bivalves, or both. Two are from the Laminarian 
zone, aud within 2 miles from shore; in the one instance, where the bottom 
was gravelly and stony, univalves prevail, and those alive ; in the other, where 
it was sandy, bivalves prevail, and those mostly dead. The remaining five 
papers are from the Coralline zone; in three of them, where the bottom was 
a scallop bank several miles from shore, the number of both bivalves and 
univalves taken alive was very considerable, reaching in one instance respec- 
tively to 21 and 27. In one, from a nullipore bottom one mile from shore, 
univalves prevail, but bivalves are also abundant. In one, on a gravelly and 
stony bottom near shore, bivalves prevail (the numbers being 24 living and 
25 dead), but univalves are also plentiful. 
Among sixty-four dredging papers from the Clyde district and the He- 
brides, twenty-two exhibit numbers either of bivalves or univalves above 10; 
of these three come within the Laminarian division, and one from depths very 
close to shore; in two of these the number of species of living univalves pre- 
vails ; in one, of the bivalves. From the upper part of the Coralline zone 
there are eleven papers, in six of which the bivalves prevail, all from muddy 
or sandy bottoms, sometimes mixed with stones, close to shore ; in two, uni- 
valves prevail over bivalves, in gravelly and stony bottoms near shore ; and 
in two, the numbers are nearly equal on stony and mixed bottoms near to 
shore. From depths between 40 and 60 fathoms, there are six prolific papers, 
all richer in bivalves than in univalves, and all from sandy, gravelly or muddy 
beds, varying from two to ten miles from shore. A bottom of gravel and 
sand in 90 fathoms, close to shore, is richest in bivalves. 
Of thirty papers from the Zetlands, sixteen are rich in species; one only 
is from the Laminarian zone, on a sandy bottom, especially rich in living bi- 
valves (30), and having many (15) univalves also. Of two, from the upper 
part of the Coralline zone close to shore, one, with a nullipore and ston 
bottom, is richest in univalves; the other, from a shelly bed, in bivalves. Of 
the thirteen remaining papers from depths between 40 and 100 fathoms, eight 
present considerable numbers of both univalves and bivalves, and in five (all 
from depths below 60 fathoms) bivalves prevail. The numbers of species of 
bivalves are high in the depths at a considerable (30 to 100 miles) distance 
from shore. The bivalves are also predominant at these great depths on more 
or less muddy bottoms, and at the farther distances; the univalves most 
numerous alive where the bottom is more or less stony. 
Gregarious and prolific species —Many of our littoral mollusca, as the 
shore-living species of Littorina, Purpura, Trochus, Cardium, Donax, 
Serobicularia, Mya, Pholas, &c., are truly gregarious, and the individuals of 
each are constantly found assembled together in considerable numbers. This 
is not so commonly the habit among sub-littoral species ; among them, how- 
ever, there are some habitually gregarious (as Ostrea edulis, Pecten opercu- 
laris, Corbula nucleus, Syndosmya alba, Pectunculus glycimeris, Modiola 
modiolus, and Turritella terebra; and among radiata, Ophiura rosula, 
Uraster rubens, Comatula europea, Echinus sphera), though with this dif- 
ference as compared with most littoral gregarious forms, that whereas the 
individuals of the latter are always assembled together, the sub-littoral species 
are gregarious in some zones of depth, and under certain conditions of sea- 
bottom, whilst they are at the same time diffused in small numbers, or even 
