OF ARTS AND SCIENCES. 39 



thallus, is free above. The growth now becomes more rapid, the disk 

 enlarges, and, at the same time growing upwards, rolls back the thai- 

 line elements, so that by the time the disk appears on the surface it is 

 surrounded by a large thalline exciple. Tlie treatment adopted in 

 other cases shows that here also both asci and paiaphyses arise from a 

 common system of hyphai forming the thin hypotheoium. (Plate III. 

 Figs. 20, 21.) It often happens that the disk enlarges very much after 

 reaching the surface, and that the exciple is thereby rolled back upon 

 the thallus. Thus the cortex enveloping the apothecium is bent back 

 upon the cortex of the thalline surface, and the former, as in P. molyhdea, 

 puts out hair-like processes which soon form an intricate hyphal layer. 

 (Plate III. Fig. 18.) There are thus formed beneath the mature 

 apothecium, except at its centre, two layers of cortex separated from 

 each other by a loose weft of hyphce. 



COLLEMEI. 



In approaching the truly homoeomeric groups of lichens, we realize 

 at once that we are now on debatable ground, for it is this group 

 which formed the basis for the theory of lichen-sexuality, the one 

 group, so far, in which the conclusions reached by Stahl and his fol- 

 lowers seem plausible. In the subfamily Eucollemei, as understood 

 by Tuckerman,* we find five genera, of which the last, the peculiar 

 genus Hydrothyria, Russell, will first occupy our attention. I take up 

 the consideration of this lichen now, instead of later, because I cannot 

 but regard the synoptical position which it at present holds as by no 

 means a settled one. Writing upon this point Tuckerman himself 

 says: " In this type (Hydrothyria), remarkable alike in its characters 

 and its habitat, Collema, Ach., which we found to reach its extreme of 

 development in the Leptogia of more recent authors, may be said now 

 to revert evidently towards Pannaria, or even Peltigera." f Instead, 

 however, of regarding this type as a reversion to a preceding type, it 

 seems more fitting, for the present at least, to consider this form as 

 following directly upon the Peltigerei and Pannariei, which in many 

 points it most closely resembles, and as foi'ming an additional link be- 

 tween the homoeomeric and heteromeric forms, while inclining decid- 

 edly toward the latter. The habit of Hydrothyria venosa, Russ., the 

 only known representative of the genus, although in some measure 

 like that of Leptogium, seems to approach much more nearly that of 



* Tuckerman, Synopsis of North American Lichens, p. 5. 

 t Tuckerman, Genera Lichenum, p. 102. 



