﻿294 Evans: The genus Plagiochasma 



carpocephala are terminal. The appendages of the ventral scales 

 are likewise very different, those of the Reboulia being narrowly 

 linear or lanceolate. 



A close relative of P. jamaicense is P. crenulatum, a somewhat 

 larger and firmer species, the aeriferous layer being a little more 

 compact. In distinguishing the species the best difTerential 

 characters are drawn from the epidermal pores and ventral scales. 

 Although the pores in the two plants are built up on the same 

 plan, those of P. jamaicense tend to be more complex and usually 

 show a greater number of radiating series of cells around the 

 opening with more cells in each series. The ventral scales are 

 much the same in their basal portions except that those of P. 

 crenulatum are larger and show a broader border of contorted 

 cells. It is in their appendages that the scales are most distinct. 

 In P. crenulatum these are broadly ovate to orbicular, the largest 

 reaching a length of 1-1.4 mm. and a width of about i mm.; in 

 P. jamaicense they are more narrowly ovate, the largest reaching 

 a length of 0.45 mm. and a width of 0.3 mm. In P. crenulatum the 

 apical contraction is usually abrupt, in P. jamaicense it is more 

 gradual, and the apex seems to be invariably tipped with a row of 

 two or three cells. Another difference to be noted is in the position 

 of the androecium. In P. crenulatum it is borne on a short 

 ventral branch and is apparently never succeeded by female 

 receptacles as is the case in P. jamaicense. Unfortunately, as 

 already pointed out, it is unwise to place too much confidence in 

 a difference of this sort. The spores of P. crenulatum, finally, 

 are more finely reticulated than in P. jamaicense, the spherical 

 face showing eighteen to twenty meshes instead of only twelve. 



4. Plagiochasma Wrightii Sulliv. 

 Plagiochasma Wrightii Sulliv.; A. Gray, Manual, Ed. 2, 688. 



pi. 6. 1856. 

 Aitonin Wrightii Underw. Bull. Illinois State Lab. Nat. Hist. 2: 



43- 1884. 



Thallus pale green and glaucous above, with a narrow purple 

 border, plane or broadly canaliculate, strap-shaped, sometimes 

 forking, sometimes innovating at the apex, sometimes with ventral 

 adventive branches, margins undulate-crenate, more or less cris- 

 pate, mostly 1.5-2 cm. long and 3-5 mm. broad, thickness of 



