17 
view oval; tumor in the centre granular; two distinct longitudinal, 
nearly parallel rows of teeth or aculei. Diameter, 75/i; length, 
loopt. 
Pond near Minneapolis, Minn, 
C. ACULEATUM, n. sp, (Plate xxvil, Fig. 12). — Medium size, subor- 
bicular, length slightly less than the diameter ; constriction deep, 
forming, by the incurving of the angles of the semicells, two ellipti- 
cal sinuses; membrane primarily more or less densely aculeated ; 
later, the aculei drop off and leave short granule-like stumps. 
Length of cell, 30// ; breadth, :^:^}J^ ; breadth of constriction, 10/^. 
Pond, Minneapolis, Minn, 
The outline of this species bears a resemblance to that of C. 
S?nolandicu?ny Lund., but it is aculeated and devoid of the papilla 
at the angles of the semicells, 
C atncenum.BxQh.y var. tumidum, n. var. (Plate xxvii., Fig. 16). 
Usually somewhat larger than the typical form ; proportions the 
same, but the sides of the semicells tumid, not " rectis parallelis," 
Occasional in ponds in Pennsylvania and New Jersey. 
C. EvERETTENSE, H. Sp. (Plate XXVIL, Figs. 25, 2^a and 25^, etid, 
side and front views). — Cell as long as wide ; constriction deep 
linear ; semicells broadly rounded at the ends, inferior angles obtuse 
and close ; membrane granular, with large verrucae arranged in 
concentric series ; apex usually nude, surrounded by short, acute, 
conical teeth or aculei; end and transverse views showing a decided 
central inflation. Length and breadth, 50/^-51//. 
Ponds, Mount Everett, Mass. 
C Brebissonii, Menegh., comes nearest, but differs in the form of 
the verrucae, their uniform shape, and their even distribution over 
the cell ; it is also without the central inflation. 
XA'NTHIDIUM, Ehrb.— X rectocornutum, n, sp, (Plate xxvii., 
Figs. 6 and 6^).^— Cell as long as wide ; constriction linear, some- 
what gaping ; semicells semicircular, finely punctate, or smooth ; 
two rows of beads above the central protuberance, the one with 6-10 
and the other under it with half the number ; another series of beads 
on the base, forming a ring around the isthmus ; ends broadly 
rounded, nude; basal angles armed with two pairs of aculei, one 
horizontal, subulate, the other vertical, straight, neither curved nor 
divergent ; having the form of an hour-glass when viewed trans- 
versely, truncate, crenate at the ends, with two vertical aculei in the 
centre. Diameter of cell, without spine, 55/^-60//. 
Frequent in ponds, Mount Everett, Mass. 
This form differs from X. antelopmnn^ Breb.,its nearest of kin, in 
having the end of the cells bare, the aculei not incurved or diver- 
gent, but straight and erect, attached to the sides and springing from 
the basal angle. It is also quite unlike that species in the three 
series of beads and in its transverse view. 
ARTHRODESMUS, Ehrb.— A. Rauii,;^. J/. (Plate xxvii., Figs. 
17 and 17^, end and front views). — Cell slightly longer than wide, 
aculeated or verrucose ; aculei short and stout, deciduous, leaving, 
after falling off, large verrucae, usually six on the margin of each 
end, and two curved series of six each on the membrane within the 
