190 THE AMERICAN MONTHLY [Juue 



Rhizomorphs differ from sclerotia in being long, cord- 

 like bodies formed also from the mycelium. These struc- 

 tures radiate from the point of origin in all directions in 

 the soil, at a depth of a few inches below the surface. 

 When the tip of a rhizomorph comes in contact with the 

 root of a tree it penetrates the tissues, where a dense 

 mycelium soon forms, feeding upon and eventually kill- 

 ing the tree, after which the above-ground fruit of the 

 fungus is developed, new rhizomorphs meanwhile radi- 

 ating from the new centre of development in search of 

 other victims. Armillaria mellea, a very common Brit- 

 ish "toadstood," spreads rapidly by means of subterra- 

 nean rhizomorphs, and proves very destructive to various 

 species of forest trees. — Q. O. 



On the Structure of the Nodules in Pleurosigma. 



EDWARD M. NELSON. 



While examining the Pleurosigmas in the Nottingham 

 (Maryland,) deposit I was much struck with a marked dif- 

 ference in the structure of the nodules when compared 

 with those of receDt forms. Now, as the Nottingham de- 

 posit belongs to the Middle Tertiary Period, it cannot fail 

 to be interesting to compare these with other fossil and 

 recent forms. 



Fig. 1 shows the raphse pipes entering the nodule of 

 a Nottingham Pleurosigma, as seen from the outside of 

 the valve. You will notice that both ends of the pipe, 

 after making a slight dip, terminate with their extremi- 

 ties pointing directly at one another. Fig. 2 is a very 

 highly magnified picture of the extremities of the pipes; 

 they are joined by a very fine thread-like structure, which 

 by the way is an excessively difficult image. Passing on 

 now to the nodule itself, we come to fig. 3 or an inside 

 view at the highest focus ; fig. 4 is the same at an inter- 

 mediate focus; focussing down still lower we come to an 



