272 DAWSON 



crustose members of the Corallinaceae because of the excessively time- 

 consuming difficulties of interpreting the literature and of preparing 

 suitable materials for study. Furthermore, it was found that for some 

 of the genera of these calcareous plants one needed to resort essentially 

 to monographic studies in order to arrive at a satisfactory interpretation 

 of the individual Mexican collections. Such a difficulty was encountered 

 in the relatively obscure genus D ermatolithon and has led, preparatory 

 to the treatment of the several Mexican species, to the following pro- 

 visional working key. 



The genus D ermatolithon Foslie has recently been discussed by 

 Mason (1953), but she dealt specifically only with two of its Pacific 

 North American members and made no effort to relate them to the vari- 

 ous other species of this poorly documented assemblage. Inasmuch as the 

 literature is so scanty on many of these plants and the distinctions between 

 them are so unsatisfactorily recorded, it appears to be worthwhile to 

 present a working key to the species as currently recognized in the litera- 

 ture, as a means of pointing out the characters by which they are supposed 

 to be distinguished. In doing this, the writer realizes that we have as yet 

 little knowledge of the relative stability or taxonomic worth of the various 

 characters used in this synopsis. It is presented with the hope that workers 

 in the various regions where these plants occur may study them more 

 carefully and record those pertinent data which may ultimately aid in 

 clarifying the circumscriptions of the natural species. 



1. Plants growing upon stones, pebbles or mollusk shells ... 2 



1. Plants growing epiphytically upon other algae 11 



2. Thalli relatively thin, mostly of 5 or fewer cell layers; 



conceptacles emergent, hemispherical 3 



2. Thalli relatively thick, of 7 or more cell layers; concep- 

 tacles sometimes prominently elevated, sometimes not . . 7 



3. Thalli with more or less extensive monostromatic margins ; peri- 

 thallium of 1 to 4 or more layers, or of similar, superimposed 

 layers (crusts) ; hypothallium cells of more or less variable 

 length 4 



3. Monostromatic margins limited; perithallium mostly 1 to 3 

 cells thick, the cells of variable length; hypothallium cells of 

 more or less uniform length (45-60 ju.) , only occasionally to 90 /a 

 long; crusts superimposed. . . . D. veleroae Dawson (1944) 

 Type locality : Gulf of California 



