1 97 



and they consist structurally of bundles of hyphae extending longi- 

 tudinally with the long axis of the thallus-lobes just above the 

 lower cortical layer. These are, doubtless, primarily a mechanical 

 tissue adapted to resist longitudinal (pulling) tensions. 



The apothecia are comparatively few, scarcely medium in size, 

 and closely resemble those of Leptogium. The spores are more 

 elongated and are not multilocular. It seems probable that this is a 

 further degenerative change in the Collemaceous spores. It may, 

 however, be probable that Hydrothyria is represented by a different 

 fungal ancestor than the preceding genera. 



Another marked peculiarity of Hydrothyria is its habitat. Nor- 

 mally it occurs upon rocks in the bottom of clear running water, 

 in which position it matures its spores. Further observations in re- 

 gard to its life-history are necessary. Its distribution is perhaps 

 much as the preceding genera. Only one species has so far been 

 reported (H. venosa Russ.) 



It is highly probable that Hydrothyria is closely related to Pso- 

 roma and Heppia. For the time being its present position seems to 

 be most nearly in accordance with our knowledge of the subject. 



It is also interesting to note that spermagonia have never been 

 observed on Hydrothyria, and this seems to be additional evidence 

 that these structures can not be the male reproductive organs, since 

 apothecia are formed without them ; they could not occur as para- 

 sitic fungi on account of the submerged habit of the plant. 



PLATE 64. 



HYDROTHYRIA VENOSA Russ. 



1. Thallus natural size. 



2. Lobe of thallus showing venation on lower surface. 



3. Section of apothecium. 



4. Section of thallus; d, vein cut transversely. 



5. Paraphysis and spore-sac. 



6. Spores. 



7. Algae. 



Family 9. PANNARIACEAE. 



The limitations of this family are at present uncertain ; cer- 

 tain genera are included which no doubt belong to some other family ; 

 , in particular, is out of place ; Solorina, Peltigera and Ne- 



