236 FRESH WATER SPONGES OF NOVA SCOTIA—MACKAY. 
I. S. fragilis. Leidy. 
Sessile, encrusting. Statoblasts in extensive basal layers, 
with apertures extended into generally curved tubules, directed 
outwards, or aggregated into compound spherules of three or 
four or more single statoblasts in a common cellular investment 
with spicules, cylindrical, roundish truncate, spined and generally 
shightly curved. 
Described first by Leidy, 1851; by Bowerbank as S. Lordi 
from British Columbia in 1863; by Dybowski as S. Siberica 
in Russia. The varying forms with aggregated or compound 
statoblasts, were described as Var. Segregata, by Potts; but he 
says a wider experience has induced him to give it up. It is 
this form which has been described from the Ottawa, by Dr. G. 
M. Dawson, in 1875, as S. Ottawaensis. His “large irregular 
ovaria” are the compound statoblasts to the structure of which 
he does not allude. These forms are frequently found encrust- 
ing submerged twigs, stones, &c., in the lakes of Nova Scotia. 
(See slides 20 and 21.) 
2. S. lacustris Var. Dawsoni, Bk. 
Generally branching. Besides the skeletal spicules there are 
present minute, spined, fusiform dermal arcuates. Statoblast 
spicules, cylindrical, spined and more or less curved. 
This is a variety of the European S. lacustris. The first 
Canadian species was described by Bowerbank in 1863 
as S. Dawsoni. Next we find S. lacustroides. Then as its 
specific identity with the European form become apparent, we 
find S. lacustris var. lacustroides Potts, and S. lacustris var. 
Americana Carter. We suggest the propriety of retaining 
Bowerbank’s specific as the varietal name. We have a specimen 
from Sir William Dawson, collected by H. M. Ami in the Ottawa, 
which is nearly identical with the European S. lacustris. 
One of the most common of Nova Scotian fresh water sponges. 
(See slides 22 and 23.) 
