276 Alexander IV. Evans, 



tacles : the first shows nine truncate rays clearly, the basal sinus 

 being broader than the others ; the second shows ten less clearly 

 marked rays. The type of M. disjuncta agrees closely with the 

 specimens distributed by Sullivant, by Austin, and by Under- 

 wood. They are well represented in the beautiful figures pub- 

 lished by Sullivant,^^ and the female receptacles show a variable 

 number of rays. The type of M. Elliottii bears numerous female 

 receptacles, which show from five to nine rays apiece, the rays 

 being plane or slightly convex and blunt. The type of M. 

 caracensis bears both cupules and female receptacles, the latter 

 showing five rays apiece. All of these specimens show the 

 dentate scale appendages and other features characteristic of 

 the species. 



The thallus of M. domlngensis is smaller and usually more 

 delicate in texture than in any of the preceding species. In wet 

 localities the photosynthetic layer is especially thin and the epi- 

 dermis is rarely more than a single cell in thickness, so that 

 plants growing under these conditions present an unusually 

 fragile aspect. The epidermal pores, however, maintain their 

 complex structure and form conspicuous whitish dots on the 

 upper surface. Sometimes the boundaries of the air-chambers 

 show distinctly through the epidermis, but they are usually indis- 

 tinct. The structure of the pores is much the same as in M. 

 breviloha, although the inner opening is bounded by straighter 

 lines. 



The sclerotic cells in the thallus exhibit a great deal of varia- 

 bility. When abundantly developed they occur both in the 

 thickened median portion of the thallus and in the wings, those 

 in the latter position showing distinctly as elongated brown spots 

 when examined from underneath. The sclerotic cells appear to 

 be separated from one another by parenchyma when a cross sec- 

 tion of a thallus is examined. As a matter of fact, in the median 

 portion of the thallus at least, tliey often form elongated strands 

 running for a considerable distance, the acute ends of the cells 

 slightly overlapping. There are many cases, however, where the 

 sclerotic cells are very scantily developed. Sometimes there are 

 none at all present in the wings although the median portion 

 still retains them; sometimes even the median portion seems to 



Mem. Am. Acad. II. 3 : pi. 3. li 



