39 
greater when the mussel is never exposed yet the sea mussel has not been found 
of recent years in the St. Andrews region below low-water datum. 
A consideration of the distribution of its chief predatory enemies shows how 
delicately adjusted is the balance determining the lower limit of the beds. Below 
low-water datum the mussels are constantly menaced by their enemies among the 
fish, and by the'starfish, sea-urchins and whelks. The mussels above low-water 
datum, when exposed by the tide, are entirely free from fish depredations and 
the menace from starfish, sea-urchins and whelks is at least greatly lessened. 
This weakening of the action of their enemies is sufficient to allow the mussels 
to form beds in the littoral zone, although there they must endure the detrimental 
effects of exposure between tides as well as the attacks of the Purpura and the 
occasional attacks of crows and gulls. The upper limit of the beds is evidently 
determined by the effect of exposure on the young mussel larvae since primarily 
the beds are formed by the development of the young mussels where they settle 
when leaving the free-swimming mode of life. Which is the detrimental factor 
acting during exposure that determines the death of the young mussel the writer 
has not investigated. It may be the action of light, heat (and consequent 
drying), or lack of food, etc., or the combinétion of all these. The upper limit 
of the beds is naturally not as sharply defined as the lower one since the protection 
against exposure is not equal in all locations and since also drifting mussels 
detached during storms may fasten themselves and survive above the upper 
limit at which the younger ones would be killed. 
In concluding, the writer wishes to-express her indebtedness to Professor A. 
G. Huntsman (under whose direction the work was undertaken) for many helpful 
suggestions during the conduct of the work, and to the various assistants at the 
Biological Station, particularly Capt. Mitchell and Engineer Bartlett, whose 
hearty co-operation was invaluable. 
LITERATURE. 
CLEMENS, WILBERT A. and CLEMENS, Lucy S. 
1921. Contribution to the Biology of the Muttonfish, Zoarces anguillaris. 
Contributions to Canadian Biology, Ottawa (69), 1921. 
DETWEILER, JOHN D. ; 
1915. Preliminary notes on the mollusca of St. Andrews and vicinity, New 
Brunswick. Contributions to Canadian Biology, 1911-1914, Fasci- 
culus 1, pp. 43-46. 
FIELD, IRVING A. 
1911. The food value of sea mussels. Bulletin of U.S. Bureau of Fisheries, 
Vol. XXIX. (1909), 1911, pp. 85-128. 
GANONG, W. F. 
1885. On the zoology of the invertebrate animals of Passamaquoddy Bay. 
Bulletin of the Natural History Society of New Brunswick, No. IV., 
pp. 87-97. 
1887. Marine mollusca of New Brunswick, Ibid., No. VI., pp. 17-61. 
1889. The economic mollusca of Acadia. Ibid., No. VIII., pp. 1-116. 
AZ 
