500 Howe : Phvcological studies 



we have identified with the Mediterranean Halimeda Tuna present 

 some slight pecuHarities which, however, we believe are not suffi- 

 ciently constant and rehable to warrant a specific separation. In 

 the first place, the American plants are more rigid and rather more 

 calcified than the European specimens that we have seen. When 

 growing, they are commonly suberect and cespitose, while accord- 

 ing to Oltmanns * the flat Halimedas of the Mediterranean (" Tuna, 

 platvdisca") have a more or less horizontal or pendulous position. 

 The diameter of the peripheral cells averages about 6-12// less 

 than in the European specimens examined, though no smaller than 

 in certain Adriatic representatives of H. Tuna. The filaments of 

 the central strand, as showTi in our figures 5 and 6. are often 

 incUned to cohere strongly at the node just above the points of 

 fusion, while those of H. Tuna are usually easily separable under 

 treatment, as described by Mrs. Gepp "f" ; however, we have seen a 

 specimen from the Adriatic (leg. Titius) in which the tendency of 

 these filaments to cohere at the nodes is as pronounced as in the 

 plants of Bermuda and Porto Rico, and Mrs. Gepp mentions (/. c.) 

 a similar condition in a specimen brought from Rangiroa by Pro- 

 fessor Agassiz. The sporangiophores in the American specimens 

 appear to be rather more simple than those of the European H. 

 Tuna, as we have noted above. 



The American species of the Halimeda Tuna group, thus far 

 recognized, may be distinguished by the use of the following key % : 



Peripheral utricles truncate or rounded -obtuse. 



Peripheral utricles in contact for \ their length or less ; utricles of the subcortical 

 layer subturbinate. obconical, or clavate, 35-Ilou in maximum diameter. 



H. Tuna. 

 Peripheral utricles in contact for \-\ their length ; utricles of the subcortical laver 

 ballate, mostly ventricose-obovoid, 1 10-2 15 a in maximum diameter. 



H. discoidea. 

 Peripheral utricles galeate-cuspidate. ff- scabra. 



*Morph. and Biol. Alg. i : 295. /. 182; 296. 1904. 



t Siboga-Espeditie. Monographe 60 : 16. 1901. 



i Excluding Halimeda gracilis Harv., which agrees essentially with the members 

 of the Halimeda Tuna alliance in the character of the nodal filaments, but differs con- 

 siderably in the form of the segments. The type of H. gracilis was from Ceylon, but 

 a specimen dredged by the Challenger Expedition at St. Thomas, West Indies, in 5 to 

 15 fathoms of water, appears to agree with it in most respecu, though the peripheral 

 utricles are larger and more coherent and the utricles of the subcortical layer are very 

 long (mostly 300 to 700 u). 



