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dilated and carrying inside near the end a small seta, dactylus rather feeble. 

 Copulative appendages with the lower terminal lappet very small, upper well 

 developed, lamellar. Ejaculatory tubes with about 18 whorls of radiating 

 spikes, proximal end rather produced, bottle-shaped. 



Colour opaque whitish grey, generally with a more or less distinct brow- 

 nish tinge. 



Length of adult female attaining 1.10 mm. 



Remarks. I think I am right in identifying the form here described with 

 that recorded by Kock under the above specific name. It has generally been 

 combined with /. gibba (Ramdohr); but I find it convenient to keep it apart 

 as a nearly allied species, since no true transitions between these 2 forms have 

 occurred to me. The specific name proposed by Koch is fairly well applic- 

 able to the present form, the 2 transverse folds of the valves being more 

 sharply marked off than in any other species known to me. 



Occurrence. I have taken this form occasionally in small ponds and 

 ditches near Christiania. In some cases it occurred in company with /. gibba: 

 but in other places only this form was met with. On the whole it would 

 seem to be less common in our country than /. gibba, whereas in other coun- 

 tries it is by far the predominant form. In habits the animal is rather sluggish, 

 being more generally found creeping slowly on the bottom more or less 

 deeply burried within the loose mud. It is however by no means devoid of 

 swimming power, as could be expected from the well-developed natatory setae 

 on the antennae. Indeed, at times it is seen moving freely in the water in a 

 peculiar abrupt manner; but these movements are never carried on for any 

 longer time, and the animal very soon sinks back to the bottom. Evidently 

 the heavy shell, which moreover is often thickly covered with mud, con- 

 siderably encumbers the free movements of the animal. Male specimens seem 

 to be very rare. Indeed, I have not come across this sex among the Norwegian 

 specimens examined, nor has it been observed by any of the Swedish authors. 

 My knowledge to the male of this species is due to some few specimens 

 which, together with numerous females, were raised, many years ago, from 

 dried mud taken in Algeria and kindly sent to me from Dr. Chevreux. 



Distribution. Throughout Europe, Algeria, North America. 



