50 Rhodora [Marcu 
fluitans is more robust than any of the other species in the subaduncus 
group, but it bears a marked resemblance to D. Kneiffii and the two 
are very liable to be confused. In D. pseudofluitans the leaves often 
reach a length of 5 mm. and throughout the plant are pretty uniformly 
ovate-lanceolate in shape, tapering gradually toward the apex. In 
D. Kneiffii the leaves almost never exceed 4 mm. in length and are 
dimorphic; of the two types of leaves one resembles those of D. 
pseudofluitans in shape, but the other is broadly ovate and tapers 
rather abruptly. Both kinds of leaves are usually present in the same 
individual. Furthermore, in D. pseudofluitans the leaves near the 
tip of the shoot have a tendency, which is usually not evident in D. 
Kneiffü, to wrap themselves more or less loosely around the stem. 
The length of stem is also said to differ, but specimens of each species 
have been collected by the writer in which the stem measured fully a 
foot in length. On the whole, however, the distinctions which can 
be brought out in a description are rather unsatisfactory for, as Loeske 
remarks,! the two forms are more readily separated by their general 
habit than through any anatomical differences. D. pseudofluitans 
sometimes grows in ditches or in shallow depressions which are dry 
during the summer, but in such habitats it is poorly developed. It 
thrives most luxuriantly in places where it is almost completely sub- 
merged throughout the year. In a locality of this sort along the lake- 
ward margin of a swamp which borders one of the Twin Lakes there 
are pure mats of D. pseudofluitans many square yards in extent. The 
range of this species has not been definitely established, but for the 
present it may be assumed that its distribution coincides approxi- 
mately with that of D. subaduncus. So far as the writer is aware 
no other New England stations have been published. 
In passing it is worthy of note that this moss, together with D. 
Sendtneri var. Wilsoni ?, which occurs in the same locality, may be of 
considerable importance locally in connection with the production 
of marl. Davis has shown? that the extensive marl accumulations, 
which are a well known feature of lakes in limestone regions, are 
largely the result of plant activity, and he has cited a number of algae 
and seed plants which play an important róle in this connection. In 
! Moosflora des Harzes 308. 1903. 
? D. Wilsoni Schimp. is often treated as a separate species, but by Warnstorf it is 
regarded merely as a variety of D. Sendtneri. 
3 Geol. Surv. Mich. 83: 65-100. 1903. 
