AND ALLIED FORMS. 31 



water. It moves under water with an awkward gliding motion, 

 very different from its active " step" on land. 



Without further preface I will proceed to the description of the 

 soft parts of the type of the genus. 



Pomatiopsis lapidaria. 



The wood-cut, Fig. 22, represents the animal as it appears 

 when placed in water. 



The foot is a very large muscular organ, ^'S- ^2. 



the texture of which resembles much more 

 that of the Pulmonates than that of Am- 

 nicola and other aquatic forms. It is con- 

 siderably broader than that of Amnicola, and 

 capable of being protruded forward some- 

 what in advance of the rostrum, notwith- 

 standing the considerable length of the latter 

 organ. In progression on land, however, the end of the rostrum 

 is constantly kept in advance. The lateral angles of the anterior 

 extremity of the foot are not sufficiently produced to form auricles. 

 Its posterior extremity is broadly rounded. The lateral surface 

 of the body and foot presents a system of sinuses adapted for the 

 peculiar mode of progression of the animal, which will be de- 

 scribed below. These sinuses are most distinctly seen on the left 

 side, to which the following description more particularly applies 

 (see Figs. 25 and 26). First there is a distinct fold separating the 

 foot into an anterior and a posterior part, the latter being about 

 twice as large as the former ; which fold, though very conspicuous 

 on the upper surface of the foot, does not distinctly appear on its 

 lower surface, nor form an emargination upon its edge, except 

 when the animal is in motion. This fold terminates above at the 

 point Avhere the foot joins the rostrum. Next, above and nearly 

 at right angles with the first fold, there is a horizontal sinus also 

 arising from the juncture of the foot and rostrum, and separating 

 the foot from the body ; — the upper margin of this fold is con- 

 tinuous posteriorly with the operculigerous lobe. Above this 

 there are two oblique folds arising from the inferior base of the 

 rostrum and extending upward and backward, the upper one 

 reaching to the base of the tentacle, and the lower one extending 

 upward along the side of that constriction of the body which is 

 sometimes called the "neck," or pedicle. The position of these 



