Difflugia oblonga Ehrenberg, 1838
Diagnosis: Shell ovoid-elongate, laterally circular, with a smooth surface, generally transparent in appearance, and 110 µm long.
Dimensions: Ehrenberg: 110 µm long; Leidy: 60-580 µm; Cash: 100-300 µm; Hoogenraad & De Groot: 110-440, sometimes up to 800 µm (pers. comm.). My measurements? See my remarks!
Ecology: in sediments of fresh water ponds, ditches and bogs; also in moist soil.
Remarks: The original description is short but clear: shell oblong (=ovoid-elongate), rounded fundus, smooth, 110 µm long and brownish.
Unfortunately this fairly general description has led to different interpretations and alternations by several authors. For example, Cash and Hopkinson (1909) modified the character "oblong" in "typically oblong or pear-shaped", and "rounded fundus" in "the crown arched, sometimes furnished with a mamillary protuberance". They argued that "although this species is now almost universally called Difflugia pyriformis, Ehrenberg's name has the priority and must be adopted in accordance with the rule of zoological nomenclature". If you are looking through the literature most species labeled as D. oblonga are pyriform (pear-shaped) and much larger than the length of 110 µm given by Ehrenberg (Fig. 1).
An other example of misinterpretation of the original description of D. oblonga can be found with Ogden and Hedley (1980) who noticed that D. longicollis Gassowsky, 1936, is similar to D. oblonga in shape, but can easily identified by its size which is 72 - 116 µm. However, tis is curious, as the size given for D. oblonga in the original description is 110 µm!
Two species meet the description of Ehrenberg, D. bryophila and D. lanceolata (Fig. 2). Very likely Ehrenberg has observed a specimen of one of these common species, but we will never now which of these species he has used for his description. However, most species identified as D. oblonga belong to the D. pyriformis group. In this regard, it is highly remarkable that Penard in his standard work (1902) never mentioned the name D. oblonga, though he used it in later years, e.g. D. pyriformis var. bryophila became D. oblonga var. bryophila (Wailes & Penard, 1911).