32 



RESEARCHES UPON THE HYDROBllN.B 



latter folds will be better understood Ij}'' an inspection of the 

 wood-cuts, Figs. 25 and 2G. It was probably the observation of 

 these sinuses which induced Dr. Lewis, in the papers already 

 referred to, to consider the species as allied to the Melanians. 

 The sinus in the side of the foot in our American Melanians is, 

 however, of a different character and connected with the sexual 

 system. 



The rostrum or snout is longer than the tentacles, and capable 

 of great protrusion. It is wrinkled transversely, and its ex- 

 tremity may be expanded so as to form an adherent disk, the 

 upper margin of which is eraarginated, while the lower side con- 

 tains the mouth. The tentacles are short, subulate, pointed, and 

 somewhat thickened near the base. They are not capable of 

 erection in air, but droop, hanging down by the side of the head 

 and resting against the base of the rostrum. The eye is situated 

 on the outer side of a rather prominent swelling out or protube- 

 rance of the head at the base of the tentacle. On the upper and 

 inner side of these protuberances there is a conspicuous longi- 

 tudinal fusiform spot of flake-white or yellow, which is a promi- 

 nent character, probably, however, of specific importance only. 



The generative organ of the male (Fig, 23) is situated nearly 

 in the median line of the back, and comparatively very 

 Fig. 23. faj. behind the head, so that it is entirely concealed 

 within the branchial cavity. It is of great size, and 

 when extended would reach to the middle of the ros- 

 trum. It is thus twice as long as in Amnicola, and 

 moreover is not bifid, but consists of a single broad 

 flattened process, convoluted in a spiral of about one and a half 

 turns, with a pointed extremity, rounded and smooth outer 

 margin, and sharp, wrinkled inner margin. 



The gill, situated in the position usual in Ctenobranchiates, is 

 rather broader than in the Hydrobiina?, and the plates, though 

 nearly similar in shape to those of that genus, are much less 

 projecting, and more broadly rounded at the summit. The 

 rectum, lying to the right of the gill, contains faeces formed into 

 little oval pellets ; and it may here be mentioned that this char- 

 acter, though seemingly of trivial importance, will distinguish 

 also all of our Hydrobiinse from the YiviparidoB, in which the 

 faeces appear in a continuous vermiform shape. 



In the manducatory apparatus we find jaws resembling those 



