NO. 1 TAYLOR: PACIFIC MARINE ALGAE 175 



MELOBESIA Lamouroux, 1812 



Plants crustose, completely attached to the support, composed of but 

 a single layer of cells, or of but few layers, and thickest in the neighbor- 

 hood of the conceptacles ; tetrasporangia zonate, in conceptacles with sev- 

 eral pores; spermatangia in conceptacles, lateral on two-celled filaments 

 distributed over the bottom of the cavities ; cystocarpic conceptacles coni- 

 cal, prominent. 



Certain of the species included below were originally described in the 

 subgenus, or genus, Heteroderma (Foslie 1900c, p. 21; 1905c, p. 102; 

 1905d, p. 8; 1909, p. 56), but the unsubstantial character of the distinc- 

 tions ascribed to this genus are pointed out by Rosenvinge (1917, p. 237). 

 Lemoine (1929, p. 59) uses the genus Epilithon for these species, which 

 Foslie also used ( 1909, p. 55), but that genus is based on the same species 

 as the genus MelobesiUj which has priority. The species below are quite 

 distinct from the Lithothamniums included here, which are substantially 

 calcified, though they have also been put in that genus (Foslie 1929). 

 Therefore, these three species have been retained in Melobesia and the 

 description of the genus relaxed slightly over the form used in describing 

 the Atlantic flora (Taylor 1937, p. 267). 



KEY TO SPECIES 

 1. At most the margin of the frond monostromatic .... 2 



1. Monostromatic over a considerable part of the thallus; mono- 

 stromatic part with cells quadrate to radially elongated and 7-11 

 IX long, 6-8 jx broad; sporangial conceptacles 150-250 /i. diam., 

 feebly convex or in the center depressed, perforated by 25-50 

 pores; c57stocarpic conceptacles conical, 140-240 p. diam,, sperma- 

 tangial conceptacles 70-100 /x diam M. galapagensis 



2. Near the margin monostromatic; marginal portion with cells 

 7-11 fi long, 4-7 /x wide, and in section 7-18 /a tall, but between 

 the conceptacles much larger, to 36 /x tall, 14 /x broad ; sporangial 

 conceptacles 150-300 /x diam., feebly convex or plane, but slightly 

 elevated, in the center depressed; cystocarpic conceptacles low 

 conical ; spermatangial conceptacles 80-100 /x diam, . M. mediocris 



2. Plants crustaceous, purplish, small, commonly confluent, lightly 

 calcified and thin, the margin translucent, of two cell layers, the 

 central part to 9 cell layers and 50-100 /x thick; cells of the basal 

 layer 12 /x long, 5-8 ju, wide, and as high or higher; the upper cells 

 6-8 /x diam., up to 10 /x tall; the tetrasporangial conceptacles 

 crowded, sometimes anastomosing, slightly convex, with the 

 central portion of the roof depressed, with numerous pores, diam. 

 150-200 /x M. marginata 



