Mr. W. Clark on Ancylus oblongus and A. fluviatilis. 281 



We hope to prove that the next species, the sinistrorsal A. 

 fluviatilis, is an undoubted congener of our present animal, and 

 that both will be acknowledged to have every essential character 

 of the Liinnei. 



The reproduction of the Ancyli, and Limneadce in general, is 

 that of the Helicida, a congressional hermaphroditism, though 

 differing in some curious particulars. Our dredger has repeat- 

 edly verified in this species the recorded accounts of the singular 

 androgenous concatenations of this family when under the ge- 

 nial infiuences, — another proof of their race, fi'om habitudes, in- 

 dependent of animal structure. 



This species abounds at Exmouth on aquatic plants. I have 

 never seen it on stones, as is generally the case with A. fluvia- 

 tilis. The animal at this season (July) is infested with fourteen 

 to twenty or more of a species of Gordius or Filaria, which are 

 slender, cylindrical, and very vivacious, blunt or rounded at 

 both extremities; they fix themselves between the mantle and 

 the body around the pedicle of the foot, and are in constant 

 vibration, — so much so, that a novice might mistake them for 

 branchial filaments. One would suppose that such a colony 

 of vigorous parasites must greatly annoy the " beneficiaire :" I 

 have examined 100, and all had this retinue. 



Ancylus fluviatilis, auct. 



The shell presents a more elevated and regular cone than that 

 of the Ancylus oblongus, and when cleansed from its ferruginous 

 coat, is of a clear, very light, delicate horn-colour, and adorned 

 with fine close-set raised strife that diverge from the apex to the 

 periphery of the aperture ; the vertex is posterior. 



The animal is so generically similar to the preceding species, 

 that it will not be necessary to do more than to present a com- 

 parative view of their organs. 



This species is sinistrorsal, but that is a mere accidental con- 

 dition involving only a transposition of the organs. The prin- 

 cipal variation as regards the shell is its more circular figure, and 

 in respect of the animal, the foot is shorter and broader, the 

 result of the less elongated cone ; otherwise it is of similar shape 

 and rounded at both ends ; the colours only difi'er in being of a 

 more quiet drab, speckled with white points, instead of the pale 

 golden-yellow hue and minute lead-coloured lines or dots that 

 prevail in the A. oblongus. 



The head, hood, veil, and buccal apparatus of the two, scarcely 

 vary, beyond the crosial orifice of the latter organ in this animal 

 being more rayed and finely puckered, within which is a similar 

 palate of two fleshy lobes rounded in front and pointed behind ; 

 from thence a rather shorter and broader tongue passes through 



