THE GREEN AI.GAE OF NORTH AMERICA 431 



ments are in many cases like those of Dichotomosiphon. As far 

 as vegetative structure is concerned, there would seem to be a 

 definite phylogenetic series from Dichotomosiplwn through 

 Avrainvillea, Penicitlits, Rhipoccphalus and Udotca to Ifalitncda ; 

 but while the reproductive organs of Dichotomosiphon are of high 

 rank, those of Halimcda are 1 much lower and of a different type, 

 and practically nothing is known as to the reproduction of the 

 intermediate genera. If any of these genera should ever be 

 found to have fructification resembling that of Dichotomosiphon, 

 or if sporangia like those of Halimcda were found in D. pusillus, 

 the latter would have to be considered as the primary form of 

 the Codiaceae or at least of the Udotoideae, from which the 

 other forms had developed. There is one well known fresh 

 water species, to which a marine form is now doubtfully 



added. 



KEY TO THE SPECIES OF DICHOTOMOSIPHON. 



i. Filaments 40-100 M diam. ; freshwater. i. D.tuberosus. ' 



i. Filaments 10-30 p diaui. ; marine. 2. D.pusillus. 



1. D. TUBEROSUS (A. Braun) Ernst, 1902, p. 115, Pis. VI-X ; 

 Vancheria tuberosa A. Braun in Kiit/cing, 1856, p. 23, PL L,XV ; 

 Wolle, 1887, p. 154, PL CXXIX, figs. 9-14, PL CXXX ; P. 

 B.-A., No. 764. Fronds 2-10 cm. long, 40-110 /* diam., usually 

 70-95 p. ; akinetes straight and elongate or clavate and curved, 

 0.5-5 mm. long, 200-400 p. diam. ; antheridia and oogonia corym- 

 bosely arranged at the ends of the ultimate divisions ; antheridia 

 cylindrical or clavate, more or less incurved, 130-170X35-50^; 

 oogonia globose, 290-320 p. diam. ; oospore globose, dark green, 

 250-280 p. diam. Fig.^ 158. Out., Pa., Mich., 111., Ga., Texas. 



Europe. 



This species occurs in similar localities to Vaucheria ; it 

 appears to be more common in America than in Europe, where 

 it is reported only from Switzerland. In P. B.-A., No. 764, the 

 oogonia are large enough to be seen by the naked eye, but are 

 erroneously referred to in the label as "tuber-like swellings." 

 Wolle's varieties intermedia and minor are merely smaller, sterile 

 forms. 



2. D. pusillus n. sp. Filamentis prostratis, irregularibus, 

 10-30 p diam., hinc et illinc coustrictis, di- trichotomis ; ramis 

 basi fortiter constrictis, apice et sub dichotomiis saepe clavifor- 

 miter distentis ; ramis lateralibus brevibus et simplicibus fre- 

 quentis, sine ordine egredientibus, raro oppositis. 



Filaments prostrate, irregular, 10-30 p diam., here and there 



