210 DESCBIPTION OF \_Polji/gastric(u, 



particles of sand, ovoid, elongated, and contracted anteriorly. Length 

 1 -325th to 3-325th. It approaches D. proteiformis, but differs in 

 its more elongated form, in being contracted anteriorly and almost 

 pyi'iform, in being sometimes depressed, and lastly in its greater 

 size : margin of aperture uneven. 



Genus SpiEiLLnsrA. — Lorica tubular, siliceous, rolled in a spiral 

 manner, resembling the shell of a Planorhis. It is allied to Dijfluyia, 

 Avith a siliceous lorica — acids exert no action on the shell. This genus 

 probably agrees with the SpiruUna of Bory St. Vincent, but the 

 latter name has been otherwise used by Ehrenberg to designate a 

 genus of Polythalamia. 



S. vivipara. — Shell porous, convoluted as a circular, spiral, hori- 

 zontal tube, hyaline and smooth. Young lorica3 may often be foimd 

 connected with it. (P. 14. f. 37.) Found living in the sea — Vera 

 Cruz, Mexico. 



The form of this species recalls that of many undoubted Polytha- 

 lamia, whilst it has no fellow among the Infusoria. Ehrenberg has 

 lilfewise represented apparent dots or pores on its surface, like those 

 tlu'ough which the filiform processes of Polythalamia are proti'uded ; 

 and the only reason implied in Ehi'enberg's account of this organism, to 

 reckon it among the Pohjgastrica, is, its siliceous shell. It will be 

 noted that Elirenberg is inclined to believe it viviparous. 



Genus Aucella. — The capmle Animalcules possess numerous 

 variable processes, or single processes, cleft into many, and spread 

 abroad, and is furnished with a flattened shield-hke lorica. The 

 structure of the lorica, as to details, is very different in the various 

 species. For instance, in A. vulgaris it exhibits regular and delicate 

 facets; in A. dentata, the facets are large and crystaUine; in A. 

 aculeata, it is beset with spicula , and in A. hyalina, it is homo- 

 geneous and clear. The organs of locomotion are extensile and 

 retractile processes, radiant and variable. The digestive cells are 

 readily filled with colom-ed vegetable substances. In A. vulgaris, 

 a contractile vesicle has been perceived. 



"The Arcellce (says Dujardin,) seem to differ among themselves 

 by the intimate structure of their lorica, which sometimes appear 

 membranous, at others finely striated, reticular, or with granules dis- 

 posed in spiral lines. Some Arcellce have also spinous prolongations 



