Diphyscium chiapense sp. nov. from Chiapas, Mexico, is described and illustrated. It is unique in the involutum group by its unistratose leaf margin, and it is unusual because of its highly mamillose median lamina cells. The bistratosity, rhizoid insertion, and axillary hairs of the Diphysciaceae are described as features reliably distinguishing these plants from the Pottiaceae. Three collections of mosses from the vicinity of Mapastepec, Chiapas, originally identified as Diphyscium foliosum (Web.) Mohr represent an undescribed species of the genus. These collections contain male and female plants and a few quite immature sporophytes. Traditionally, the more diagnostic features in the genus are gametophytic (including perichaetial) characters, and mature sporophytes tend to be quite stenotypic, not usually useful in species definition. For this reason, I do not hesitate to describe a new species on the basis of largely gametophytic characters. Diphyscium chiapense Norris, sp. nov. FIG. 1-12 Species haec ab Diphyscio involuto Mitt. differt marginibus unistratosis, cellulis medianis valde mamillatis, foliis oblongo-lanceolatis. Small plants with stems to 1.5 mm long, essentially unbranched; rhizoids arising from a circular cluster of initials at the abaxial base of costa, coarsely striate to papillose, sparsely branched; stem leaves oblong-lanceolate to lingulate-spatulate, 1.8-2.2 mm long and 3-6:1 in male plants, 4.2-5.4 mm long and 5-8:1 in female plants, rounded to minutely mucronate at apex; costa strong but with indistinct margins, broadening abruptly at the base and filling 2/-3A of that base, ending at apex or minutely apiculate; median laminal cells rounded quadrate, 9-10 /m wide, strongly mamillose on both surfaces; upper marginal cells smaller and somewhat flattened against the margin, forming a slightly and irregularly crenulate border; upper lamina bistratose except near margin where it becomes unistratose; basal cells thin-walled and rectangular, hyaline, unistratose, with non-vertical end-walls; base of leaf gradually narrowed to the costa, not at all auriculate; axillary hairs polytrichoid and decorticated (see text of this paper). Dioicous. Outer perichaetial leaves strongly differentiated, to 5.5 mm long, with costa broader than in the vegetative leaves, upper laminal cells reduced in extent, and with basal laminal cells proportionately increased in number; inner perichaetial leaves progressively reduced in size with concomitant reduction of lamina cells to extinction in the innermost leaves; costa strongly excurrent in all perichaetial leaves with the apex of the lamina sagittate and sometimes slightly fimbriate; perigonial leaves somewhat shorter than adjacent vegetative leaves but otherwise undifferentiated; archegonia and antheridia numerous within their respective enclosing structures; paraphyses absent. 007-2745/81/375-378$0.55/0 This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:49:01 UTC All use subject to http://about.jstor.org/terms 376 BRYOLOGIST [Volume 84