224 FRESH- WATER RHIZOPODS OF NORTH AMERICA. 



body of sarcode, more or less including the lower part of the nucleus. In 

 individuals which would appear to have been in an active state, the lower 

 portion of the sarcode mass was mingled with variable proportions of globules 

 or vacuoles of liquid of different sizes. In a similar position in active indi- 

 viduals, the colorless globules were mingled with distinct food globules of a 

 brownish hue, and with them occasionally an alga of some kind, as seen in 

 figs. 1, 4. 



The nucleus is large and clear, and occupies the usual position in 

 the fundus of the sarcode. Mostly, it could be seen to contain a central 

 nucleolus, or a greater and variable number scattered throughout. 



In the more active individuals, from one to three contractile vesicles 

 would appear and disappear in the intermediate zone of the sarcode. 



Many individuals exhibited the sarcode contracted into a mass occu- 

 pying the upper part of the shell, as seen in figs. 7, 9. In these the mass 

 was perfectly quiescent, and consisted of a uniform basis of finely granular 

 protoplasm with diffused oil molecules and the large clear nucleus. Some- 

 times the mass appeared to be relieving itself of a portion of its granular 

 contents by expulsion in layers, as represented in fig. 9. In some speci- 

 mens the protoplasmic mass was enclosed in a thin homogeneous membrane, 

 and the mouth of the shell was closed by compression, as seen in figs. 7, 8. 



Among many empty shells of Placocista spinosa, exhibiting considera- 

 ble variation in exact form, especially in the oral pole, I observed several, 

 as seen in fig. 14, which contained the empty lenticular shell of an egg-like 

 body evidently belonging to the animal. 



The ordinary size of Placocista spinosa is about 0.125 mm. in length, 

 0.833 mm. in breadth, and about half that thickness. The mouth is 0.05 

 mm. wide, and the spines range from 0.02 mm. to 0.033 mm. in length. 

 The nucleus of the sarcode measures about 0.0285 mm. in diameter. 



ASSULLNA. 



Latin, assxila, a little chip. 

 Difflugia ; Euglypha; Assulina: Ehrenberg. 



Shell compressed spherical or oval; neck almost obsolete, terminating 

 in a transversely elliptical mouth with uneven or ragged edges, composed 

 of minute oval or hexagonal plates in alternating rows. Sarcode and pseu- 

 dopods as in Euglypha. 



