ARCELLA VULGAEIS. 121 



not frequently extended beyond the outer margin of 

 the test. Contractile vesicles, numbering four or more, 

 situated near the outer margin of the endoplasm. 



Dimensions : Diameter of test variable, averaging 

 70 /a or more ; the mouth 12-20 fi. 



Common and universally distributed, most frequent 

 in the ooze and amongst surface vegetation in ponds. 



The colour of the test varies from a clear hyaline or 

 pale yellowish tint (in the young state) to a deep brown. 

 The membrane is thin and pliable, becoming brittle 

 with age. Conjugation has been observed to take 

 place, accompanied by fusion of the protoplasm, but 

 this is not of frequent occurrence. " Budding " is 

 more common. 



Var. angulosa (Perty) Leidy. (Plate XV, figs. 12 



and 16.) 



Arcella angulosa PERTY Kenntn. kleinst. Lebensf. (1852), 

 p. 186; MAGGI in Atti Soc. Ital. XXI (1878), pp. 314,315. 



Arcella vulgaris var. angulosa LEIDY Freshw. Rhiz. N. Amer. 

 (1879), t. xviii, ff. 8-13. 



Arcella costata (Ehrenb.) PENARD in Rev. Suisse Zool. IX 

 (1901), p. 237, and Faune Rhiz. Leman (1902), p. 401, ff. 



Test smaller than in the type, faceted, forming a five- 

 to eight-sided figure, with obtuse angles; the angles 

 rising obliquely from the broad base to the narrower 

 depressed summit. Mouth central; protoplasm, nucleus, 

 and contractile vesicles as in the type. 



Dimensions : Diameter of test averaging 30-40 /a. 



Occurring occasionally in marshy places, amongst 

 Sphagnum, and in ponds. 



Var. gibbosa (Penard) G. S. West. (Plate XV, figs. 11 

 and 17; and fig. 19 in text.) 



Arcella gibbosa PENARD in Mem Soc. Phys. Geneve, XXXI, 

 no. 2 (1890), p. 155, t. v, ff. 96-99 ; t. vi, f. 1. 



Arcella vulgaris var. gibbosa Gr. S. WEST in Journ. Linn. 

 Soc., Zool. XXVIII (1901), p. 314, and op. cit. XXIX 

 (1903), p. 109. 



