GENUS DIFFLUGIA— DIFFLUGIA CONSTKICTA. 121 



A. ffomceochlamt/8 htnala. Ehrenberg: Ibidem, 244, 274. 

 A. Heterocosmia Arctiscon. Ehrenberg : Ibidem, 245, 274. 

 A. Bclerocosmia guatimalensis. Ehrenberg: Ibidem, 245, 274. 

 Arcclla borealis. Ehrenberg: Nordpolarfahrt, 1874, Taf. iii, Fig. 29. 

 Arcella latlceps. Ehrenberg : Ibidem, Fig. 30. 



Shell laterally ovoid, with the fundus posterior and more or less pro- 

 longed obliquely upward, obtusely rounded and simple, or in the largest 

 forms often provided with from one to half a dozen conical spines. Mouth 

 anteroinferior, large, circular or oval, and inverted, with the anterior lip 

 often prominent. Shell as usually seen (lying on the front, by transmitted 

 light) more or less pyriform, with the narrower part downward and including 

 the mouth, which appears as a clearer transversely oval or somewhat reni- 

 form or circular space ; sometimes in the shorter forms nearly circular or 

 even transversely oval in outline. 



Shell composed of hyaline quartz-sand, or of chitinoid membrane, 

 usually with variable proportions of scattered mineral particles. Colorless, 

 yellowish, or brown. Interior sarcode transparent and colorless. 



Size. — Spineless specimens range from 0.09 mm. long by 0.078 mm. 

 broad, to 0.232 mm. long by 0.16 mm. broad. The spine-bearing forms 

 range from 0.18 mm. long by 0.12 mm. broad, to 0.34 mm. long by 0.18 

 mm. broad.* 



Locality. — The smaller spineless forms are found almost everywhere 

 in moist places; the larger forms, including those bearing spines, are found 

 commonly in the ooze of ponds. New Jersey, Pennsylvania, Maine, Flo- 

 rida, Alabama ; and at Fort Bridger and in the Uinta Mountains, Wyoming 



Difliiigia constricta, of which many forms are represented in pi. 

 XVIII, is one of the most common species. It holds a slanting position in 

 comparison with that maintained by others ; that is to say, when the ani- 

 mal is erect, as in its ordinary movements, the long axis, corresponding with 

 a line passing from the centre of the mouth to the summit of the shell, is 

 oblique instead of being perpendicular. The inclination of the axis ranges 

 between 30° and 60°. 



Commonly the shape of the shell is slightly compressed pyriform or 

 ovoid, with the narrower end downward and forward. It is of variable 



"The length is taken from the anterior lip to the fundus, as the specimens are usually seen lying 

 on the object-glass of the microscope. The true length would be from the centre of the mouth in the 

 axis of the shell to the fundus. 



