304 EVANS. 



closely that it would be difficult to tell them apart if these features 

 alone were relied upon. Both species, for example, show a flat or 

 nearly flat thallus, branching both dichotomously and ventrally, and 

 having a costa bounded on each surface by two rows of cortical cells. 

 The costae, moreover, although usually naked, occasionally develop 

 a few ventral hairs ; the alar cells are almost identical in size and in the 

 characters derived from their walls; and the marginal hairs exhibit 

 a similar range in abundance, being sometimes numerous and some- 

 times very few and occasionally showing a twinned arrangement 

 although usually occurring singly. 



On the whole M . epiphylla (Fig. 8, A) is slightly smaller than M. 

 dccipiens and prefers living leaves as a habitat, although it occasion- 

 ally grows on bark. M . dccipiens, on the contrary, is much more at 

 home on bark and other substrata than on leaves. One other vegeta- 

 tive difference to be noted, although more observations are necessary 

 to prove its constancy, is the occasional presence of ventral alar hairs 

 in M . epiphylla and their complete absence in M. dccipiens. 



The most trustworthy differential characters, however, are those 

 derived from the sexual branches and the capsules, and these are sup- 

 plemented by the presence of gemmae in M. epiphylla and their absence 

 in M. dccipiens. The male branches are much alike in the two species, 

 except that those of M. epiphylla are even more strongly incurved, so 

 much so that the apex usually comes in contact with the base. The 

 female branches when normally developed are distinguished by their 

 greater hairiness, the ventral hairs not being restricted to the thickened 

 median portion, as in M. dccipiens, but scattered over the entire sur- 

 face. The capsules are mainly distinguished by differences in the 

 character and distribution of the local wall-thickenings of the valves. 

 In M. dedpiens, as has been shown, the median wall of the outer layer 

 has two rows of local thickenings, these being largely restricted to the 

 inner longitudinal walls of the valve-cells. In M. epiphylla the 

 median wall has no local thickenings or very small ones (Fig. 8, B), 

 approaching in this respect the condition described by Andreas. On 

 each side of this median wall two rows (or more rarely three) have the 

 thickenings restricted to the outer longitudinal walls, while the remain- 

 ing cells have them on the inner walls, as in M. dccipiens. Each valve 

 thus has two longitudinal walls with double rows of local thickenings. 

 In the inner layer of the valves, transverse bands, instead of being 

 conspicuous, are either lacking or very indistinct (Fig. 8, C), although 

 the prolongations of such bands on the radial walls are still apparent. 



Even in the absence of sexual branches and capsules, the presence 



