OF WESTERN PENNSYLVANIA. 49 



. Family II. DICRAXACBAE. 



Autoicous or dioicous ; large to minute, mostly cespitose: 

 stem with a central strand, often thickly covered with rhizoids, 

 mostly densely leafy, branched ; leaves often falcate-secund, 

 mostly acuminate to narrowly linear from a broader base, 

 usually more or less smooth and shining, usually costate ; costa 

 sometimes clorsally serrate, heterogenous ; leaf-cells sometimes 

 mamniillate, the basal ones enlarged and mostly transparent, 

 alar ceils often much larger and either hyaline or brownish, 

 the central leaf-cells short to rounded, mostly smooth; 

 perichcetial leaves often sheathing: seta usually erect and long; 

 capsule mostly unsymmetric, usually cernuous, when dry often 

 curved and silicate; annulus present or absent; peristome 

 simple or rarely none ; when present the peristome teeth are 

 16 in number, approximate, united below into a basal mem- 

 brane, usually two-parted to the middle, or beyond, into linear 

 T awl-like divisions, no longitudinal lines, but the teeth 

 minutely striate or papillose on the dorsal face, rarely smooth, 

 inner face yellow with one or two longitudinal lines and with 

 more or less projecting trabeculae; operculum more or less 

 long-rostrate; calyptra usually cucullate. 



AYy to the Genera. 



a. Cells of costa in cross-section homogeneous; peristome-teeth 

 broad, flat, undivided, mostly smooth, rarely none. 



6. Seligci'ia. 



a. Cells of costa as seen in cross-section heterogeneous; teeth nar- 

 row, prolonged, mostly two-parted, striate or papillose.' 



b. 



b. Alar cells not differentiated. i. Bruchia. 



b. Alar cells differentiated. c. 



c. Cells of the lamina smooth. d. 



c. Cells of the lamina mamillate on the free surfaces. 



d. Capsule mostly long-necked; stomata numerous in the neck 



or in middle of capsule-wall. e. 



d. Capsule with a short neck or none; stomata few or none. 



g. 



e. Calyptra mitrate: operculum either not deciduous or else not dif- 

 ferentiated, f. 

 e. Calyptra cucullate: operculum deciduous. 2. Trematodon. 

 f. Capsule immersed, erect, without a neck, apiculate. 



(Sporledcra) 

 f. Capsule immersed or slightly exserted. with a distinct neck, 



more or less cernuous, rostrate. I. Bruchia. 



g. Capsule mostly un symmetric, not erect. 7. Dicranella. 

 g. Capsule symmetric and erect. h. 



h. Operculum not differentiated. 3. Pleuridium. 



h. Operculum differentiated. i. 



i. Upper areolation rounded-quadrate: capsule plicate or furrowed. 



nor plicate. 4- Ditrichum. 



i. Upper areolation more or less elongate: capsule neither furrowed 

 nor plicate. 5- Ccratodon. 



