﻿298 Evans: The genus Plagiochasma 



where the margin is vaguely denticulate. These broad appendages, 

 however, seem to be always associated with narrower appendages 

 of the usual type and the apices show a much greater degree of 

 variation than in P. jamaicense, being sometimes acute and some- 

 times obtuse. 



Although P. Wrightii is doubtless a close relative of P. crenula- 

 tum the two species differ strikingly in appearance. P. crenulatum 

 is not only a larger plant but it looks much firmer and more com- 

 pact owing to the smaller intercellular spaces in the aeriferous layer. 

 It also lacks the waxy covering on the cuticle, which is sometimes 

 conspicuous in P. Wrightii and gives the thallus a glaucous aspect. 

 The margin is about as crenulate in P. Wrightii as in P. crenulatum 

 but the whole marginal region is thinner and more markedly 

 crispate. With respect to branching, P. crenulatum rarely, if ever, 

 develops a jointed thallus, while such a condition is usual in P. 

 Wrightii. This difference, however, as already emphasized, is 

 one which must be used with caution. 



5- Plagiochasma Landii sp. nov. 

 Thallus pale green above but not glaucous, with a narrow 

 purple border, plane or broadly canaliculate, strap-shaped, rarely 

 forking, usually innovating at the apex or with ventral branches, 

 margins undulate, slightly crispate, i cm. or less in length, 4-7 

 mm. in width, thickness of thallus about one sixth the width; 

 epidermis composed of thin-walled cells with small but distinct tri- 

 gones and a thin smooth cuticle, the cells averaging about 28 /x in 

 diameter; pores slightly elevated, rather small, surrounded by 

 five to seven radiating series of cells with two or three cells in each 

 series, radial walls slightly thickened ; aeriferous layer about three 

 fourths the thickness of the thallus in the middle, fairly compact 

 with the air spaces about as wide as the green cells or a little wider, 

 tending to extend at right angles to the epidermis; ventral scales 

 broadly lunulate, purple, entire, cells along the margin smaller, 

 more or less elongated parallel with the margin and with scattered 

 evanescent papillae, cells containing oil-bodies few and scattered, 

 the scales gradually narrowed into a single subulate, acuminate 

 appendage, varying greatly in length, entire or rarely with a sharp 

 tooth, scales sometimes with two such appendages: inflorescence 

 monoicous (paroicous): 9 receptacles usually borne singly, pe- 

 duncles very short, usually about i mm. long, scales of the carpo- 

 cephalum lanceolate, acuminate, somewhat contracted at the 



