OLYPEOLTNA MAfeGINATA. 127 



appropriates one of the old outer valves and secretes 

 one new valve ; thus in any individual the two valves 

 are of different ages. Diatoms are its principal food 

 but small algas are also ingested. The contractile 

 vesicle pulsates very slowly, having a period of many 

 hours. 



Sub-family ALLOGROMIINJ:. 



This sub-family was founded by Rhumbler in 1903 

 to include those genera belonging to the Reticulosa, 

 that is having pseudopodia forming a network or 

 reticulum, which up to that time had been included in 

 the family Gromida (Clap. & Lachm., 1859) whose 

 type was Gromia oviformis Duj. 



Rhumbler's reason for this alteration was that G. 

 oviformis (a marine species) is drawn by Dujardin 

 with filose pseudopodia, and cannot therefore be 

 taken as the type for a group of reticulous species. 



It is not unlikely that Dujardin failed to see the 

 full development of the pseudopodial reticulum as this 

 IB usually formed gradually, a number of filose pseudo- 

 podia being first protruded, which branch in various 

 directions, coalescing with one another until an intri- 

 cate network is eventually formed. This point of 

 view is supported by Grueber's drawings of Gromia 

 lagenoides and G. mollis as well as Dujardin' s of G. 

 fluvialis (fig. 157), all of which show very immature 

 reticula, nearly all the pseudopodia being simply filose. 



However as Dujardin neither depicted nor described 

 a pseudopodial reticulum for G. ovi/ormis, the classifi- 

 cation of Rhumbler has been adopted by us. 



The following description by Leidy (' Freshw. Rhiz. 

 N. Amer.' p. 279) of G. terricola* gives an excellent 

 idea of the appearance of the reticulated pseudopodia 

 characteristic of this sub-family : " Over each and 

 every thread of the pseudopodal net Gromia has as 



* = Allogromia fluvialis. 



