284 EVANS. 



from the marginal hairs, which are described as "hamate," and from 

 the costa. The latter is said to be strongly convex dorsally and nearly 

 plane ventrally and to have a thickness of five cells. The dorsal 

 cortical cells, furthermore, are said to be convex and much larger than 

 the internal and ventral cortical cells, which are said to be subequal 

 in size. Unfortunately the type specimen does not support this 

 description very convincingly. The marginal hairs are very rarely 

 hamate, most of them being straight or irregularly curved and con- 

 torted; while the costa, as shown by cross sections, may project 

 ventrally slightly more than dorsally. The dorsal cortical cells, more- 

 over, measure about 50 /jl in width, the ventral about 40 /j., and the 

 internal cells, which may be in more than three layers, measure about 

 30 /jl. These observations show that Stephani's differential characters 

 are far from constant, and yet with the withdrawal of these his descrip- 

 tions of M. longiseta and M. decrescens are almost identical. 



Although the first two synonyms of M. decrescens are placed by 

 Stephani among the Pinnatae, the third, M. Dusenii, is placed among 

 the Furcatae. It is based upon material collected by Dusen on Deso- 

 lation Island and the three original specimens examined have all been 

 badly mixed with M. frontipilis. Stephani's description of M. Dusenii 

 would seem to indicate that the species was much less variable in its 

 costal features than M. decrescens, since the cortical cells are said 

 to be in four rows both dorsally and ventrally. It is added that the 

 dorsal surface is convex and the ventral smooth, that costal hairs are 

 lacking, that the dorsal cortical cells are large and projecting, and that 

 the ventral cells are much smaller. The original material shows at 

 once the inconstancy of these features. Although some of the thalli 

 show four rows of cortical cells on both surfaces, deviations from this 

 number are frequent; some of the branches, for example, show only 

 two or three such rows, while five rows of ventral cells were observed 

 in at least one instance. Costal hairs, moreover, can be demonstrated 

 by careful search in spite of their great infrequency, and they are really 

 not much rarer than in the type of M . decrescens. The costal cells, 

 finally, show deviations from the description. In a series of sections 

 examined by the writer, the costa was found to be distinctly convex 

 ventrally, while the ventral cortical cells measured 38 fj. in width and 

 were thus only slightly narrower than the dorsal cells, which measured 

 42 fx. Aside from the characters which have just been discussed the 

 description of M. Dusenii agrees in all essential respects with that of 

 M . decrescens, since the thallus is said to be strongly convex with 

 naked wings and long marginal hairs borne singly, while the alar cells 



