504 Howe : Phycological studies 



Halimeda simidans and has some of the other minor peculiarities 

 of the true H. tridens. 



In its nearly sessile plants and in the form of its segments, 

 Halimeda simidans sometimes outwardly resembles H. Tuna, but 

 its segments are much thicker and more rigid than in that species 

 and they commonly have marginal crenulations or sometimes 

 short lobes which are wanting in H. Tuna ; and the behavior of 

 the filaments of the central strand at the nodes is very different in 

 the two species. On Culebra Island, Halimeda simulans, H. 

 Tuna, and H. tridens were found growing within short distances 

 of each other, and no intermediates were discovered. On the out- 

 side shore of the outmost of the Bogue Islands, Montego Bay, 

 Jamaica, on January 7, 1907, H. simidans was found growing side 

 by side with H. Tuna, while within a few rods were H. tridens and 

 H. Monde, and no forms showing any intergrading among these 

 four species, as we prefer to call them, were observed. From 

 Frozen Cay, Berry Islands, Bahamas, the only Halimeda that we 

 brought away besides the H. simidans was one of H. scabra, but 

 Halimeda tridens is in general rather common throughout the 

 Bahamian archipelago. 



A key to the American representatives of the Halimeda tridens 

 group may be arranged as follows : 



Filaments of the central strand coherent at the nodes, communicating there with those 

 adjacent by pits or very short tubular processes, rarely (now and then in H. 

 Monile) merely thick-walled at the nodes and separable ; segments flattened or 

 subterete, often 3-lobed or 3-dentate. 

 Peripheral utricles less than 80 /i in average maximum diameter, surface view. 



Peripheral utricles 49-77 // in average maximum diameter, surface view; seg- 

 ments usually flattened. H. tridens. 

 Peripheral utricles 30-44 /i in average maximum diameter, surface view. 

 Segments mostly subterete. t^. Monile. 

 Segments discoid (suggesting H. Tuna). H. simulans. 

 Peripheral utricles I75-I90;U in average maximum diameter, surface view. 



H. favulosa. 



D. On the sporangia of Avrainvillea nigricans 

 (Plate 28, figures 8-25) 



On January 5, 1907, at Montego Bay, Jamaica, while washing 

 and preparing some specimens of Avrainvillea nigricans and A. 

 longicaidis (A. Ma.zei) which had been collected there in shallow 



