248 Howe : Phycological studies 



is sufficiently differentiated from that by its flat, compact, crusta- 

 ceous, more or less calcareous thallus, with determinate outline 

 and radio-marginal growth. The type of the genus and the only 

 species known to the writer is 



Petrosiphon adhaerens sp. nov. 



Forming light-green suborbicular patches, mostly 2-6 cm. in 

 diameter, on rocks, often irregular in outline on an irregular sub- 

 stratum : margins radially striate or sulcate to the naked eye both 

 when living and when dried, everywhere most closely appressed 

 and adnate to the substratum ; older central parts sometimes 

 attaining a thickness of 3-5 mm.; tubes mostly 300-580 fi in 

 diameter, those of periphery often nearly straight and sparingly 

 subdichotomous, those of older parts commonly somewhat gen- 

 iculate ; cells y 2 -20 times longer than broad, cell-walls mostly 

 6 to 35 ft thick, conspicuously lamellate and often showing also 

 transverse striae on surface ; vertical tubes in central portion, 

 when well developed, springing from a comparatively thin hypo- 

 thallus, this commonly consisting of a single layer of horizontal 

 tubes transformed by the vertical elongation of nearly all their 

 component cells : calcification strongest in the vertical contact 

 planes, the walls in these planes remaining rigid on drying while 

 remainder of exposed wall of surface cells collapses, the surface 

 of thallus becoming thereby radio-sulcate toward margin and 

 usually spongiose-alveolate toward center : special fibular cells 

 very rare, adjacent cells, however, often connected by horn-like or 

 subcorneal processes : ventral rhizoids very numerous, rock-bor- 

 ing, tortuous, branched, septate, mostly 9-27 fx in diameter : cysts 

 (aplanospores) extremely variable in size and form. (Plate 15). 



Common in the Bahama Islands, growing on surf-beaten calcar- 

 eous rock near low-water mark and in tide-pools. Our no. 3322, 

 collected January 23, 1905, in tide-pools on Silver Cay, near 

 Nassau, from which our published photograph was taken, we 

 consider the type of the species. 



The thicker central portions of the thallus of this plant (exclu- 

 sive of rhizoids) can be readily removed from the rock with a knife, 

 but no specimen adequately representing the thinner marginal parts 

 can be obtained without including also the rock substratum. The 

 delicate rhizoids described above often give a green color to the 

 rock for a depth of 1-2 mm. They are not ordinarily seen at all 

 unless the subjacent rock is carefully decalcified together with the 



