﻿Evans: The genus Plagiochasma 



297 



pores, in the ventral scales, in the length of the peduncle, in the 

 scales of the carpocephalum, and in the spores. Those drawn from 

 the length of the peduncle, from the 

 scales of the carpocephalum and from 

 the spores are perhaps a little untrust- 

 worthy and difficult to apply, but the 

 others will serve to distinguish the two 

 species. In P. Wrightii the pores are 

 much the same as in P. crenulatum, 

 although perhaps a little less elevated ; 

 the opening is usually surrounded by 

 six radiating series of cells with two or 

 three cells in each series. In P.jamai- 

 cense there is a tendency at least to- 

 ward a greater complexity, many of the 

 pores showing eight radiating series of 

 cells with three or four cells in each 

 series. Of course here, as in so many 

 other cases, it is unsafe to base conclu- 

 sions on the study of only a few pores, 

 because those of P. Wrightii sometimes 

 show as many as eight radiating rows 

 while those oi P . jamaicense may show 



the radial wall-thickenings and the 

 trigones are better developed in P. 

 Wrightii than in P. jamaicense. Ac- 

 cording to Miss Haynes the append- 

 ages in P. Wrightii are less constricted 

 and narrower than in P. jamaicense 

 with their margins always entire. A 

 study of a considerable amount of 

 material show^s that these differences 

 are usually, but not always, present ; 

 there are rare cases, for example, where 

 the appendages in P. Wrightii are 

 broad enough to be called ovate, where 

 the basal constriction is marked, and 



