LAG EN Wl ALES 965 



of a purely morphological nature is necessary in order clearly to charac- 

 terize and properly place the genus. 



SIROLPIDIACEAE 



Thallus endobiotic, holocarpic, without a specialized vegetative sys- 

 tem, unbranched or with rudimentary branches, at maturity typically 

 forming a linear series of sporangia, walls giving a cellulose reaction, 

 contents undergoing strong vacuolization; sporangia with a single 

 discharge tube (occasionally several); zoospores biflagellate, monopla- 

 netic, formed in the sporangium; resting stage unknown. 



So far as is known, an exclusively marine group, parasitic and 

 saprophytic in green and red algae, and in mollusk larvae. 



The thallus, although more highly developed than that of the Olpid- 

 iopsidaceae, is, under natural conditions, very rudimentary in character. 

 In both Sirolpidium and Pontisma the mature thallus may consist of no 

 more than a single cell and it then strongly resembles Olpidiopsis. 

 Typically, however, a linear septate type is formed which parallels that 

 found in the chytrid Septolpidium. 



KEY TO THE GENERA OF THE SIROLPIDIACEAE 



Thallus typically narrowly tubular, at maturity becoming septate, the 



fragments disarticulating Sirolpidium 1 , p. 965 



Thallus broadly tubular, often with short, irregular branches, not dis- 

 articulating Pontisma, p. 970 



SIROLPIDIUM H. E. Petersen 



Oversigt Kgl. Danske Vidensk. Selskabs. Forhandl. 1905 (5): 480 



(Fig. 77, p. 967) 



Thallus endobiotic, holocarpic, without specialized vegetative struc- 

 tures, consisting either of a simple saclike body formed directly from 

 an expanded endobiotic zoospore or more commonly of an unbranched 

 or occasionally branched hypha-like tube, contents at first vacuolate, 

 later with numerous refractive granules; sporangium formed from the 

 saclike body or from fragments of the linearly septate progressively 



1 See recently described taxon, p. 968. 



