ARCELLA. 25 



GENUS XI. ARCELLA. 



Shell composed of chitinoid membrane, never of ex- 

 trinsic elements, with a minutely hexagonal cancellated 

 structure, translucent and commonly of a brown color, 

 variable in shape, but usually more or less campanulate, 

 with a circular base, concavely infundibuliform and con- 

 vex at the border, and with the mouth central. Sarcode 

 occupying the central portion of the shell, connected 

 with the mouth by a cylindrical neck, and by means of 

 threads of ectosarc with the dome of the shell. Pseudo- 

 pods few, blunt, digitate, simple or branching. Com- 

 monly two nuclei, one on each side. Contractile vesicles 

 several, occupying the periphery. 



Synopsis of the Species of Arcella. 



Shell hemispherical or bell-shaped, height half the PAOH. 



breadth, widest at the base, Vulgaris, 25 



Shell discoid or shield-shaped, height one-fourth or 



one-third the breath, Discoides, 26 



Shell mitriform or balloon-shaped, height exceeding 



the diameter of base ; widest near the middle, Mitrata, 26 

 Shell viewed from above circular and dentate, from 



the side crown-like ; breadth more than twice the 



height, Dentata, 27 



Shell basal-border everted, rising to % or % the 



height of the shell, obtusely angular ; from % to 



% as high as broad, Artocrea, 27 



Arcella vulgaris. SYN. : A. vulgaris, dentata, A. sticholepis 

 vulgaris, A. Homoeochlamis angulosa, Ehr. ; A. hemispherica, angulosa, 

 uiridis, Perty ; Arcellina vulgaris, Carter. 



Shell hemispherical or campanulate ; height about 

 half the breadth, widest across the usually slightly ex- 



