NO. 10 DAWSON : MARINE ALGAE, GULF OF CALIFORNIA 273 



cells, 9-14 fi diam., without heterocysts; cystocarpic conceptacles im- 

 mersed or forming a slight prominence, 60 /a diam. ; tetrasporangial con- 

 ceptacles unknown. 



Type: D. & R. 3328a (Field Museum; isotype, AHF no. 24), on 

 Corallina, in intertidal pools, cove north of Cabo Arco, Guaymas, Dec. 

 19, 1939. 



This species resembles Heteroderma Gibbsii but differs in habit and 

 host, and particularly in cellular structure. The cells of H. Gibbsii are 

 considerably smaller and often more flattened. The crusts are 10-30 /ut 

 thick, while those of H. corallinicola are commonly 25-40 jx thick and 

 often reach 50 /x. 



The present species would fall under Foslie's subgenus Pliostroma, 

 while H. Gibbsii he places in subgenus Euheteroderma. 



Genus POROLITHON Foslie 

 Porolithon sonorensis sp. nov. 

 Plate 57, Figs. 17-19; Plate 61, Fig. 2 



Frondes primo laminis prostratis flabellatis, imbricate superpositis plerumque 

 150-200 n crassis, et marginibus rotundis, compositae, aetate provecta laminarum, 

 undulatarum, assurgentium, erectarum usque ad 1 cm. altarum, contorte anastomo- 

 santium crustam scabrosara densam formantes; heterocystis in stratis libratis in 

 partibus crassioribus frondorum prostratorum; conceptaculis tetrasporangiiferis 

 immersis aut subprorainentibus, 175-225 \i diam., poro singulo; tetrasporangiis 

 paraphysibusque e basi tota conceptaculorum orientibus. 



Thallus at first composed of prostrate flabellate plates growing imbri- 

 cately over one another, these mostly 150-200 jx thick, with rounded, 

 light-colored margins; later, the plates becoming erect to about 1 cm. 

 high, forming a rough crust of dense, erect undulate, intricately anasto- 

 mosed flat structures ; in tetrasporic plants, coalescence of individual thal- 

 lus parts very complete and little erect development apparent except as 

 upfolds or undulations of the prostrate parts; cells of central tissue of 

 erect parts 20-25 }i long, 7-10 /x wide; cells of surface layers square or 

 oblong, 6-9 /i, diam.; heterocysts present, mostly in horizontal rows in 

 thicker parts of prostrate thallus; tetrasporangial conceptacles immersed 

 or subprominent, often forming rounded excrescences, 175-225 /t diam., 

 ovate in longi-section or with a slightly flattened floor (disk), opening by 

 a single pore without a projecting rim; tetrasporangia and paraphyses 

 arising from whole surface of the conceptacle floor ; cystocarps and anther- 

 idia unknown. 



Type : D. 226a, on rocky shore at low tide, west side of bay, Puerto 

 Refugio, Angel de la Guardia Island, Jan. 26, 1940. Herb. AHF no. 25. 



