278 



EVANS. 



from the margin. Occasional hairs of this type are to be expected on 

 almost any thallus, but it is usually difficult to detect them, except in 

 section, on account of the revolute wings. When such hairs are more 

 abundant forms are produced like Dusen's specimens from the Rio 

 Azopardo, which Stephani referred to M. pvbescens. A comparison 

 of these specimens with European material of M. pvbescens shows at 

 once that the two plants are not the same. In the Rio Azopardo 



specimens, the greater part of 

 the ventral surface of the 

 wings is still free from hairs, 

 and the thallus shows the 

 marked convexity characteris- 

 tic of M. frontipilis. 



The plants from San Pedro 

 Island, upon which Stephani 

 based his M. brcrialata, repre- 

 sent a more aberrant type. 

 According to his description 

 the most important features of 

 this species are the following: 

 a large costa, bounded both 

 dorsally and ventrally by ten 

 rows of cortical cells on the 

 main axes and by four rows 

 on the ultimate branches; nar- 

 row wings, unistratose through- 

 out and only slightly decurved, 

 ten cells wide on the ultimate 

 branches but only five cells 

 wide on the main axes; and 

 an abundance of hairs on both 

 surfaces of costa and wings. 

 The plants show that the de- 

 scription is essentially correct in most respects except that the numbers 

 of cortical and alar cells are less definite than is implied. The thalli 

 exhibit, however, two marked discrepancies, when compared with his 

 description. In the first place the wings are not always unistratose 

 throughout but may be bistratose at their junction with the costa; and 

 in the second place the wings, except when very narrow, are distinctly 

 decurved and show a broad band next to the costa entirely free from 

 ventral hairs. The latter, to be sure, are abundant and show the 



Fig. 1. Metzgeria frontipilis Lindb. 



A, B. Transverse sections of rather 

 slender thalli, X 100. A was drawn from 

 a specimen collected on Desolation Island 

 by Dusen, No. 159 in part; B, from the 

 type material of M. brerialata. 



