84 THE NORTH AMERICAN SLIME-MOULDS 



ing the plasmodium ascends preferably living stems of small 

 bushes, herbaceous plants, or grasses, and forms the aethalium 

 around the stem some distance above the ground. The cortex 

 varies in amount, is also deciduous, so that weathered or imper- 

 fectly developed forms probably represent the var. cormita, S. 

 cornuta Schum. 



2. Didymium (Schrad.} Fr. 



1797. Didymium Schrad., Nov. Gen. Plant., p. 20 (in part). 

 1829. Didyminm (Schrad.) Fries, Syst. Myc., III., p. 113. 

 1875. Didyiniitm (Schrad.) DeBy., Rost, Versuch, p. 13. 



Sporangia distinct, stipitate, sessile or even plasmodio- 

 carpous, never aethalioid ; the peridium thin, irregular in dehis- 

 cence, covered with a more or less dense coating of calcareous 

 crystals ; columella more frequently present ; capillitium of 

 delicate threads, simple or sparingly branched, extending from 

 the columella to the peridial wall. 



The genus Didymium, as set up by Schrader I.e., included 

 a number of species now assigned to Didcrma. For the 

 present limitations we are indebted to Fries and DeBary. 



The genus is among Myxomycetes instantly recognized by 

 the peculiar form of its calcareous deposits, stellate crystals 

 frosting usually distinct sporangia. 



Key to the Species of Didymium. 



A. Fructification plasmodiocarpous. 



a. Capillitium with adherent vesicles . . i . D. complanatum 



b. Capillitium simple 2. D. arellus 



B. Fructification normally distinct. 



a. Sporangia sessile or nearly so ; outer calcareous wall conspicuously 



developed 3. D. crustaceum 



b. Sporangia plainly stipitate. 



i. Peridium much depressed ; umbilicate below. 



* Stipe white 4. D. squamulosum 



** Stipe black. 



f Small, about .5 mm 6. D. minus 



ft Larger, about .75-1 mm. . 5. D. melanospermitm 

 fft Sporangia discoid 7. D. clavus 



