GENUS AEOELLA— ARCELLA DENTATA. ] 77 



ARCELLA DENTATA. 



Plate XXX, figs. 10-19. 



Arcella dentata. Ehrenberg: Ab. At. Wis. Berlin, 1800, 40; 1831,90; 1871,204. Infusionsthierchen, 1838, 

 134, Taf. ix, Fig. vii (the first figure only of the series). — Leidy : Proc. Ac. Nat. Sc. 1874, 14 ; 

 1876, 56. 



Arcella stcllaris. Perty : Mittheil. Naturf. Gesells. Bern, 1849, 126. 



. 1 reella Olctii. Perty : Kennt. hleinst. Lcbensforruen, 1852, 182, 186, Taf, ix, Fig. 4. 



Arcella stellata. Ehrenberg: Microgeologie, 1854, 192. Ab. At. Wis. 1871, 261, Taf. iii, Fig. 10. 



A. Homaochlamijs dentata. Ehrenberg: Ab. At. Wis. 1871,244. 



A. Heterocosmia stellata. Ehrenberg: Ibidem, 245. 



A. Sticholcpis stcllaris. Ehrenberg: Ibidem, 244. 



Shell, as seen from above or below, circular and more or less dentated ; 

 in the side view, crown-like; breadth more than twice the height; dome 

 convex and even, or depressed at summit and broadly fluted at the sides; 

 base centrally inverted, concavely infundibuliform, at the periphery more 

 or less everted, and divided into points of variable length. Mouth circular, 

 entire. Sarcode resembling that of Arcella vulgaris. 



Size. — Breadth between points of base from 0.132 mm. to 0.184 mm. ; 

 breadth of base to position of eversion 0.10-1 mm. to 0.14 mm.; height from 

 0.44 mm. to 0.048 mm.; width of mouth from 0.04 mm. to 0.044 mm.; 

 elevation of mouth above the base 0.01 mm. to 0.016 mm. 



Locality. — Same as for Arcella vulgaris, but comparatively rare. The 

 most characteristic found at Lake Hattacawanna, Morris County, New 

 Jersey. Obtained also in Pennsylvania. 



The dentated form (figs. 10-19, pi. XXX) only of the Arcella dentata 

 of Ehrenberg I have regarded as characteristic, while the others I have 

 viewed as subordinate forms of Arcella vulgaris. While the latter appear 

 to me as comparatively slight modifications from the evenly convex form 

 of A. vulgaris, perhaps produced by unequal traction of the threads of the 

 sarcode mass on the shell, the former could only be produced from the 

 same form of A. vulgaris by evolution or growth of the dentate processes. 



Perty has figured and described the same form from Bern, Switzer- 

 land, under the name of Arcella Okeni and A. stcllaris. 



Arcella dentata, represented in figs. 10-19, pi. XXX, occurs in the 

 same localities as A. vulgaris, but is comparatively rare. I have found it 

 in the ditches below Philadelphia, and sparingly in a number of other 

 places. The most extreme forms I found, in considerable numbers, in 

 Lake Hattacawanna, Monroe County, New Jersey. 

 Iii RUIZ 



