42 PROCEEDINGS OF THE AMERICAN ACADEMY 



become smaller, and the veins therefore are less prominent, until at 

 the extreme edge the ramifications are very delicate and anastomose 

 at various points. 



The structure of the thallus, then, though less gelatinous in its qual- 

 ity than that of Pannaria rublginosa., would bring Hydrothyria into 

 more or less close relationship with that genus. The presence of a 

 well-defined cortex investing both surfaces of the thallus would give to 

 Hydrothyria a position in our system farther removed from Leptogium 

 and Collema than is Peltigera, — a position more nearly approaching 

 that occupied by Nephroma, — while the character of the gonidia, on 

 the other hand, would bring it into relationship with Pannaria and 

 Heppia. While, then, Hydrothyria is to be considered in no sense as 

 Collemaceous in its type, it becomes a matter of great difficulty to say 

 exactly where it should be placed. It would seem fair to consider it 

 as a member of the Pannaria type serving to connect that genus with 

 the Peltigerei. 



The development of the fruit is not an easy matter to follow. 

 Spermogonia have not as yet been known to occur, nor have I been 

 able to discover any sign of such organs, though I have examined ma- 

 terial collected at different seasons of the year and in many localities. 

 Material collected near New Haven, Connecticut, in June, 1889, has 

 proved the best for the study of apothecial development. On these 

 specimens the fully developed fruit is rarely seen in quantity, nor in 

 dried specimens is it easy to detect the young stages. In fresh, moist 

 material, however, the young apothecia may be seen with a hand-lens 

 occupying in considerable numbers the extreme edges of the expanded 

 thallus lobes. The marginal position of the fruit is a rule to which I 

 have as yet seen no undoubted exceptions. It is true that a mature 

 apothecium is occasionally found at some distance from the margin, 

 but by examining younger stages it will be readily seen that these 

 have arisen at the bottom of a sinus such as frequently extends deeply 

 into a thallus lobe, and that by the marginal growth of the thallus 

 around and beyond the young apothecium, the latter comes to occupy 

 a position considerably removed from the margin. The young apo- 

 thecia usually arise at a point on the margin of the thallus where the 

 fine ramifications of the veins meet and anastomose, and if sections be 

 made at such jwints it is a mere matter of time and patience to find 

 the youngest stages. These are at first observable only as a slight 

 change in the ordinary thalline structure of the extreme margin. The 

 hypha? are seen to be even more densely interwoven than normally, 

 the usually very thin margin has thereby become slightly swollen, and 



