﻿286 EvAxs: The gexus Plagiochasma 



the pores, the appendages of the ventral scales, often double, 

 long-acuminate from a broad base, papulose on the margin. 

 The papulose margin is unfortunately no more pronounced than 

 it sometimes is in specimens of P. rupestre from other localities, 

 and the other differences, even if they were constant, would not 

 be of much significance (see Fig. 2, M, N). 



The characteristics of P. elongatum, according to Stephani, 

 are the following: the monoicous (paroicous) inflorescence, the 

 very minute pores with only two or three cells around the opening, 

 the trigones in the epidermal cells, the semilunate scales abruptly 

 contracted into one or, rarely, two long, narrowly lanceolate and 

 attenuate appendages, and the shortly emarginate carpocephala 

 bearded below by lanceolate or filiform scales. A study of the 

 type-specimen, however, shows that the pores are almost in- 

 variably surrounded by four cells (Fig. i, H), and although the 

 appendages for the most part agree with the description (Fig. 

 3, R), some of them are subulate rather than lanceolate, the con- 

 traction is more gradual than is implied, and even the narrowest 

 appendages are occasionally duplicated in European material of 

 P. rupestre. The other characters, even if constant, would scarcely 

 be differential. 



In proposing Aytonia Evansii as a new species Miss Haynes 

 compared it with P. elongatum and naturally laid a good deal of 

 emphasis on Stephani 's description. She points out differences 

 between the two species in size, in the structure of the epidermal 

 pores, and in the form of the ventral scales. She separates her 

 species because the pores are bounded by four or five cells instead 

 of by two or three, and because the scales taper gradually instead 

 of abruptly into the appendages. It has just been shown that 

 the characters upon which she relied are subject to considerable 

 variation in P. elongatum and that this species cannot be kept 

 apart from P. rupestre. It therefore becomes necessary to reduce 

 Aytonia Evansii to a synonym of the same species. At the same 

 time it is a pleasure to recommend the careful and accurate 

 description and figures in Miss Haynes's paper to students of the 

 Hepaticae. 



In P. mexicanum the original authors apparently had only 

 slight confidence. They say that it is closely allied to P. rupestre 



