THE GREEN ALGAE OF NORTH AMERICA 121 



The peculiar mode of producing the zygospores places this 

 species in a separate genus from forms like Zygnema pectinatum, 

 which it otherwise much resembles. Zygncma Ralfsii Wolle, 

 1887, p. 227, appears to be a doubtful determination, and the 

 specimens on which it is based might prove to belong to this 

 species. Of Z. aequale Wolle says "we assume the name at a 

 hazard " ; and of Z. parvulum " we quote it without confidence 

 in its value " ; these names need hardly be retained. Z. pur- 

 purcum Wolle, 1887, p. 224, is very incompletely described ; 

 neither from text or plate can be learned the form of the chroma- 

 tophores, nor the character of the membrane ; dimensions also 

 are uncertain, and until further evidence it must be classed as 

 doubtful. 



Family 2. MESOCARPACEAE. 



Cells cylindrical, always several times as long as broad, 

 united in unbranched filaments, rarely having short, rhizoidal 

 branches. Chomatophore an axile plate with several pyrenoids ; 

 nucleus at the middle of the plate. Conjugation between either 

 two cells of the same filament or cells of two filaments. Part of 

 the protoplasm separates from the rest in the cell ; the part so 

 separating unites with a similar part from the other cell, to 

 form the zygospore, which is either entirely in the tube con- 

 necting the cells, or extends somewhat into one or both cells ; 

 spore varying in form, but usually lentiform, with round or 

 four- or six-angled outline ; outer membrane of spore colored 

 yejlow or brown, smooth or variously sculptured. Partheno- 

 spores, akinetes, and aplanospores sometimes occur, much as in 

 the Zygnemaceae. 



Only-two genera of Mesocarpaceae are now recognized, Gona- 

 toncma with two species, and the large genus Mougeotia, includ- 

 ing the former genera Mesocarpus, Crater ospermum, Plagiospcr- 

 mitin, Staurospermum and Sphaerospermum . The distinctions as 

 to shape and surroundings of the spore, on which these genera 

 were founded, have been proved to be uncertain, the types 

 characteristic of the different genera sometimes occurring on the 



same plant. 



KKY TO THE GENERA OF MESOCARPACEAE. 



Aplanospores with double membrane formed in the middle part of a 

 cell, shutting off the remainder of the cell by a wall. Conjugation un- 

 known. 2. GONATONEMA. 



Zygospores formed as described for the family ; aplanospores excep- 

 tional, i. MOUGEOTIA. 



