40 PROCEEDINGS OF THE AMERICAN ACADEMY 



Nephroma. It is however more decidedly erect and foliaceous than 

 that lichen, the broad, undulate, fan-like lobes being frequently 3 cm. 

 wide, and contracted below into a stalk which serves to attach the plant 

 to the substratum. Frequently several such fronds are found growing 

 together in a dense tuft, the expanded portions floating freely in the 

 water, and often they form a dense growth covering the rocky bottom 

 of the brook in which they occur over an area several feet in extent. 

 Although at times, when a frond is pressed down upon the rocky sub- 

 stratum, it puts out from its lower surface a growth of rhizoids by 

 which it becomes attached to the rock, in its normal condition, as far as 

 my observation goes, the frond floats erect in the water, attached only 

 by the short stalk-like contraction of the base referred to above. At the 

 point where this stalk expands into the frond it gives rise to a number 

 of veins which, in a manner analogous to that seen in Peltigera, spread 

 out over one surface of the thallus, seldom anastomosing, however, ex- 

 cept where they reach the free edge of the thallus-lobe or frond. 



It will be seen, then, that Hydrothyria, while resembling Leptogium 

 superficially in its rich olive-brown color, presents on closer examination 

 points, even of habit, which are in no degree analogous to that genus. 

 This fact is emphasized when we come to study the thallus in detail. 



In considering the structure of the thallus, I will dwell at greater 

 length upon those points which are indicative of the synoptical posi- 

 tion of this lichen, — the general character of the thallus, the pres- 

 ence or absence of a cortex, and the nature of the algae forming 

 the host. If we examine a radial section of the thallus, we find that 

 its structure near the centre differs materially from that near the 

 edge. In the former case the hyphte are very large (5.6 [x to 7.3 fx. in 

 diameter), and in the lower part of the thallus maintain a compara- 

 tively regular course parallel to the surface of the thallus. Owing to 

 their large size and the comparative regularity of their course, the in- 

 dividual hyphae may often be traced to a considerable distance, the 

 interweaving and branching being not such as to cause much complex- 

 ity of structure. In fact, the structure of this layer is much like that 

 seen in the lower part of the thallus of Pannaria molyhdea. On the 

 lower surface this layer is bounded by a cortex consisting of a single 

 row of cells which are differentiated from the cells immediately above 

 them by their thicker and slijjhtly brownish walls. This simple struc- 

 ture of the cortex recalls a like condition seen in Nephroma. The 

 central part of the thallus is occupied by the wide algal zone, composed 

 of dense aggregations of small bluish-green cells, among which the 

 hyphae run in inextricable confusion. So irregular is their course as 



