OP ARTS AND SCIENCES. 41 



they traverse the algal zone, that the section has alnaost the appearance 

 of a parenchymatous tissue, in the midst of which lie the groups of go- 

 niilia with no apparent regularity. On the upper surface we find the 

 thallus bounded by a simple cortex developed from the medullary hy- 

 phce, and in no way distinguishable from that which covers the other 

 surfivce. (Plate V. Fig. 31.) It is almost impossible, so dense is the 

 tissue, to determine from sections what is the nature of the gonidia, or 

 even whether they are unicellular or filamentous. On crushing the sec- 

 tion the alg;e are freed with some difficulty from the investing hyphoe, 

 and appear unicellular. Their shape, however, is not at all that of the 

 ordinary unicellular gonidia. They are extremely irregular in outline, 

 and generally present one or more flattened surfaces, as though they 

 had originally been members of a moniliform series. By exercising 

 great care I was finally able to separate a few groups of these cells 

 without their becoming entix'ely disintegrated, and they then appeared 

 as filaments composed of three to seven cells very similar in size and 

 arrangement to the gonidia of Pannaria molyhdea, though much more 

 distorted by reason of the greater density of the investing tissue. 

 (Plate V. Fig. 33.) They are undoubtedly of the Scytonema type, 

 a fact which, if the nature of the host is to be considered of any im- 

 portance in determining the systematic position of a genus, would bring 

 Ilydrothyria into relation with Pannaria rather than with Leptogium, 



Althouch the algse are more or less restricted to a certain zone of 

 the thallus, this zone is so wide that the thallus cannot certainly be 

 called heteromeric, but seems to approach much more nearly the par- 

 tially homocomeric character of the Pannaria thallus, while, on the 

 other hand, it is not truly homojonieric if we consider Leptogium as a 

 type of such a thallus, nor does it partake of the gelatinous character 

 of the Collema thallus. Toward the edge of the thallus, even the 

 slight regularity seen in the arrangement of the hyphfe in the older 

 portions gradually disappears, and the margin of the thallus presents 

 in section only a very intricate texture of delicate hyplia?, occupied 

 throughout by the alga?. The veins present essentially the same 

 structure throughout. The hyph;e composing them arise as branches 

 of the medullary hyphoe, and, pursuing a regular radial course, soon 

 become united into firm bundles. The individual hypha; become con- 

 siderably increased in size near the point where the bundle arises from 

 the medulla of the contracted base of the thallus frond ; thus the bundle 

 itself encroaches upon the thalline elements lying between it and the 

 surface, and the latter becomes elevated in the form of a ridge or vein. 

 (Plate V. Fig. 31.) As we approach the edge, the bundles of hyphae 



