WITH DESCRIPTION OF A NEW BRACHIONUS. 61 



Lange * and Prof. Powers. t Should these changes in B. satanicus 

 be confirmed, it will be the first record of true dimorphism in the 

 genus Brachionus. Fig. 2e and f represent variations in the 

 shape of the posterior spines of the smaller form. 



Fig. 2a-f were drawn from my own preparations by Mr. F. R. 

 Dixon-Nuttall, to whom I am greatly indebted for these accurate 

 and beautiful drawings. 



The large form fig. 2a measures 408 /x (l/62nd inch), and 

 the small form 250 /x (1/1 00th inch), in both cases including 

 the posterior spines. 



Asplanchna Silvestrii, Daday. 

 PI. 5, figs. 19. 



This fine and rare species was first described by Daday in 

 1902,J and found by him in plankton collections made by 

 Dr. Silvestri in 1899 in the Lago di Villa Rica in Chile. I have 

 not been able to ascertain if this lake is brackish or not, 

 Prof. Daday having no information on this point, but the 

 presence therein of Pendalion fennicum seems to make it highly 

 probable, for the latter species has never yet been found in 

 fresh water. 



In the collections from Devils Lake I found Asplanchna 

 Silvestrii in great abundance, and moreover it presented a marked 

 dimorphism, and even polymorphism, for all gradations from 

 plain saccate forms to fully developed double-humped animals 

 were represented in the same gathering. PI. 5, figs. 1 4 

 represent three of the forms. It is not possible for me to say 

 w T hich of these forms appears first, or which is hatched from the 

 resting-egg, and what causes these changes of form. According 

 to the observations of Prof. J. H. Powers, of Nebraska Univer- 



* Zur Kenntnis von Asplanchna Sieboldii, Zool. Anz. Bd. 38, pp. 433-441, 

 November 1911. 



f A case of Polymorphism in Asplanchna simulating mutation. 

 American Naturalist, Vol. XL VI., 1912. 



\ Beitrage zur Kenntnis der Siisswasser Mikrofauna von Chile. Ter- 

 meszetrajzi Fiizeteh, 1902. 



