The Myxomyceles of the Miami I'alley, Ohio. 143 



This order is readily distinguished from the Physaracese 

 by the absence of lime from the threads of the capillitium. 



Table of Gknera of Didymiace^. 



a. The lime on the wall of the spora?igiion in the form 



of minute stellate crystals. 



1. DiDYMiUM. vSporaiigiuni simple, subglobose and stipi- 

 tate, the base commonly umbilicate, or sometimes sessile and 

 plasmodiocarp. 



2. Spumaria. /Ethalium composed of numerous elongated 

 irregularly-branched sporangia, closely compacted together 

 and confluent. 



b. The lime on the ivall of the sporangium consisting of 



miyiute roundish granules. 



3. Diderma. Wall of the sporangium with the outer cal- 

 careous layer usually compacted into a smooth continuous 

 crust. 



4. Lepidoderma. Wall of the sporangium with an outer 

 layer of large scales, consisting of bicarbonate of lime. 



I. DIDYMIUM, Schrad. Sporangium simple, subglobose 

 and stipitate, the base commonly umbilicate, or sometimes 

 sessile and plasmodiocarp ; the wall a thin membrane with an 

 outer layer of minute stellate crystals of lime. Stipe present 

 or sometimes wanting ; the columella mostly conspicuous, 

 sometimes thin or obsolete. Capillitium of very slender 

 threads, straight or often sinuous, stretching from the colu- 

 mella to the wall of the sporangium, simple or outwardly 

 sparingly branched at a sharp angle. Spores globose, viola- 

 ceous. 



Didymium, together with Spumaria, is to be distinguished 

 from all other genera of the Myxomycetes by the covering 

 of stellate crystals, like hoar-frost, upon the outer surface of 

 the sporangium. 



