298 EVANS. 



other cases, however, a prolonged search is necessary before any hair* 

 at all can be demonstrated, and there are many intermediate condi- 

 tions between these extremes. A thallus, in fact, may produce hairs 

 abundantly in one part and be hairless or nearly so in another. "When 

 the marginal hairs are crowded a single hair usually arises between 

 every two marginal cells, but sometimes the hairs arise in pairs more 

 or less frequently. ^Yhen borne singly they are either truly marginal 

 or slightly displaced to the ventral surface. The hairs are usually 

 straight and measure 0.15-0.3 mm. in length by 10-12 // in width. 

 In rare instances they are branched at the apex and act as organs of 

 attachment. Costal hairs are usually exceedingly rare, and in many 

 individual thalli none at all can be demonstrated, as Stephani notes 

 under M. glaberrima. When they occur they are either scattered or 

 in small irregular clusters and are essentially like the marginal hairs. 



The alar cells vary considerably in size (Fig. 6, A-D), not only in 

 different thalli but also (in some cases at least) in different parts of 

 the same thallus. In Spegazzini's material from Staten Island, for 

 example, the cells in most places averaged about 4S X 36 n, while a 

 branch of a thallus yielding these higher measurements in its other 

 parts had cells averaging only 35 X 29 /*. Taking the mean averages 

 of fourteen specimens the cells measure about 38 X 29 n, the highest 

 average being 48 X 26 ll and the lowest 31 X 22 ji. Stephani's 

 measurements of M. glaberrima, 36 X 36 ll, agree closely with the 

 general average. It must of course be remembered that individual 

 alar cells may deviate rather widely from these average measure- 

 ments. The cells have thin or slightly thickened walls, and trigones 

 are either minute or absent altogether. 



The male branches of M. <h cipiens present few distinctive features. 

 They are almost globular in form, the wings being involute and the 

 costa so strongly incurved that the apex almost reaches the base 

 (Fig. 6, E). The largest example measured was about 0.35 mm. in 

 diameter. Hairs are entirely absent, but the usual slime-papillae 

 are of course present. 



The female branches (Fig. 6, F-I) are broadly obovate and vary 

 from plane to convex when viewed from the ventral surface. Exclu- 

 sive of the hairs they are usually 0.3^3.4 mm. in length and 0.45-0.75 

 mm. in width. Along the margin the hairs are crowded but appar- 

 ently never in pairs, each hair representing the outgrowth of an ordi- 

 nary marginal cell. On the ventral surface the hairs, if developed at 

 all, are restricted to the thickened median portion, where from one 

 to perhaps twenty may be present, the number being usually larger if 



