454 BULLETIN OF THE BUREAU OF FISHERIES. 
Kjellman’s limitation of the littoral region has not proved altogether satisfactory, 
since many marine alge range far above the highest tide mark, especially along coasts 
wet by the spray from heavy surf, and other species are able to live in water that is 
brackish or, indeed, actually fresh. Rosenvinge (1898, p. 189) pointed out that the 
upper boundary of the littoral region should be considered as that level at which 
marine algal vegetation begins, and Bérgesen (1905, p. 709) agrees with this view. 
The littoral region can then best be defined as the zone extending from low-water mark 
to the highest point where marine alge cease to grow. As a matter of fact, however, 
in sheltered waters the upper limit will generally coincide closely with high-tide mark. 
The line between the littoral and sublittoral regions is not always easily deter- 
mined, for conditions vary in different localities. It is not safe to limit arbitrarily the 
upper boundary of the sublittoral to the lowest water or neap tide mark, for many 
species characteristic of the sublittoral will grow a little above such a line. Rosen- 
vinge and Borgesen agree in placing the boundary between the littoral and sublittoral 
somewhat above the lowest tide mark. It is probably very near to the average low- 
water level. 
The lower limit of the sublittoral region varies greatly in its depth from the sur- 
face and can not be defined with exactness. It merely marks the gradual diminution 
of vegetation until a bottom is reached that is devoid of plant life. There is no sharp 
line showing the lower boundary of the sublittoral, such as defines its upper limit at 
low-water mark. Consequently there is no line marking the upper limit of an elittoral 
region or depth from which plant life is absent. Indeed, to speak of an elittoral region 
is to use a negative expression, and the term is not important in descriptive studies on 
the distribution of alge. 
The line of greatest significance in determining regions of marine vegetation is 
that between the littoral and sublittoral, near the level of average low water. Above 
and below this boundary the life conditions differ more than at any other point between 
the upper and lower limits of marine algal life. Exposure to the air, to rain, and to 
the heat and drying influence of untempered sunlight introduce very important fac- 
tors in the littoral region which are not present in the sublittoral and make this line 
of separation a most significant one. For these reasons the littoral and sublittoral 
regions are natural divisions, and further subdivisions are of far less import and, indeed, 
can hardly be made under ordinary conditions, although some authors have attempted 
to define a supralittoral region above the littoral. 
Certain of the Cyanophycee and Chlorophyceze and a few of the Phzophycee and 
Rhodophycee are most commonly found only in the upper region of the sublittoral 
either just below the lowest tide mark or in shallow water. For these a separate zone 
might be distinguished; but there are so many species of the Pheophycee and Rho- 
dophyceze which are present in both shallow and deep water that the limits of such 
a zone, at least in the Woods Hole region, is not easily determined, since there is a 
very complex overlapping of species. For these reasons we have not attempted to 
separate and designate regions of the sublittoral further than to qualify the term with 
the words ‘‘ upper’ or ‘‘lower” in certain instances where species are very clearly 
restricted in their habits. 
When the alge of the littoral and sublittoral regions are studied closely, certain 
groups of species will be found in more or less close companionship, with definite rela- 
