282 EVANS. 



above is 43 X 33 ju. At the same time one specimen gave an average 

 as high as 55 X 35 /z, so that Stephani's figures are not excessive. 

 The lowest average obtained was 36 X 32 //. It should be remembered 

 in this connection that, in species of Metzgeria, considerable variation 

 in the size of the cells is to be expected, not only when different thalli, 

 but also when different parts of the same thallus are compared. Data 

 derived from cell-measurements must therefore be used with caution 

 in distinguishing species. 



The costa of M. decrescens (Fig. 2, B-E) yields some of the most 

 distinctive characters of the species. It not only shows the marked 

 variation in the number of cortical cells, to which attention has already 

 been called, but it often shows in addition another feature unusual in 

 the genus. In most of the Metzgeriae the transition between the uni- 

 stratose wings and the multistratose costa is very abrupt, a condition 

 clearly shown in many of the published figures. When, however, the 

 costa is large and complex, as it is for example in M. frontipilis, the 

 transition between the wings and the costa may be more gradual, a 

 narrow band of large cells two or three cells thick being interpolated 

 between the unistratose portion of a wing and the costa. In M. 

 decrescens, although the costa is not particularly complex, both types 

 of structure occur, just as they do in M. frontipilis. In some of the 

 branches the abrupt transition is present (Fig. 2, E), in others the 

 gradual transition (Fig. 2, B-D), and the latter is not necessarily 

 associated with the more complex costae. When there is a gradual 

 transition this is usually clearly apparent even when an intact thallus 

 is examined. Under these circumstances the costa seems to be 

 poorly defined, and the bistratose or tristratose band becomes evident 

 by careful focusing, the outlines of the cells in the superimposed 

 layers not corresponding. 



The type material of M. decrescens shows male branches in abun- 

 dance but no female branches. The male branches, which seem to be 

 the only ventral branches present, have involute margins and are 

 strongly incurved, although the apex does not approach the base very 

 closely (Fig. 2, F). They measure about 0.45 X 0.3 mm. in well- 

 developed examples and are wholly destitute of hairs, the only appen- 

 dicular organs developed being the slime-papillae (Fig. 2, G). The 

 alar cells are more delicate than those of a vegetative thallus and 

 average only 25 //. in diameter. In some of the other specimens 

 studied a few female branches with calyptras were found. These 

 organs bore scattered hairs and attained a length in some cases of 

 3-3.5 mm., the diameter ' being 0.6-0.8 mm. Unfortunately the 



