SCOTTISH FRESHWATER RHIZOPODS AND HELIOZOA 91 



II. RHIZOPODS AND HELIOZOA FROM THE SCOTTISH 

 FRESHWATER PLANKTON. 



About twenty species of these animals were met with 

 during the investigation of the plankton of a number 

 of lochs in the north-west of Scotland and the Outer 

 Hebrides. Rhizopods were not very numerous in the 

 plankton of any of the lakes, and in some the only 

 specimens observed were empty shells of testaceous species 

 which had doubtless been carried down into the lakes by 

 mountain streams. I have mentioned all those observed in 

 the collections, although few of these animals can be regarded 

 as plankton-forms, the only ones found at all frequently in 

 the living state being Arcella vulgaris, CypJwderia ampulla, 

 Trinenia cncJielys, EuglypJia alveolata, and E. ciliata. Of 

 these five species, very fine specimens of CypJwderia ampulla 

 and EuglypJia alveolata are by no means uncommon in the 

 plankton. 



Concerning Rhizopods which occur in the plankton, 

 Cash 1 remarks that " the open waters of a lake can hardly 

 be considered their natural home." This is true of most of 

 these animals, but there are certain species which undoubtedly 

 occur constantly in the plankton, thriving well in the surface 

 waters of lakes. Moreover, Nebela bicornis, described for the 

 first time in this paper, is only known to occur in the 

 plankton. 



The table on the following page is a list of the species 

 observed in the plankton. Those species in small capitals 

 occurred in quantity and with a sufficient percentage of living 

 specimens to be considered as true plankton-forms. 



ClatJirulina elegans occurred in large quantity in Lochs 

 Luichart and Rosque, Ross. 



NEBELA BICORNIS, sp. n. Shell rather small, from the 

 front view triangular- pyriform, lateral margins and apex 

 generally slightly convex, the two upper angles each pro- 

 duced into an outwardly diverging, obtuse process or horn, 

 which may be straight or curved and is scarcely attenuated ; 

 close to the mouth with parallel margins ; mouth very slightly 



1 J. Cash in "Jour. Linn. Soc. Zool." xxix. 1904, p. 219. 



