HELISOMA. Iiy 



Gyraalus parvus, Say, par'-vus. Shell very flat, 

 consisting of a coil of the fine, horn-like shell-tube. 

 Whorls about four in number, and the whole only 

 one-eighth of an inch in diameter. From Oregon 

 and California. 



Menetus opercularis, Gld., Men'-e-tus o-per-cu-la'- 

 ris. This species, likewise, has a flattened, coiled 

 shell, sometimes nearly plane above, with an oblique 

 aperture and a conspicuous umbilicus ; it is about 

 one-fourth of an inch in diameter. 



Helisoma ammon, Gld., He-li-so'-ma am'-mon, is 

 shown in Fig. 105. The shell of this fresh-water 

 mollusk, which is found in the San Joaquin river, is 

 in the form of a flattened tube, 

 coiled horizontally so that a cup- 

 shaped depression is left on either 

 side. As the animal grows it winds 

 its shell round and round in the 

 same plane, and does not build in 

 Fig. 105. the spiral form like most of the mol- 



lusks. The aperture is large and ear-shaped. The 

 outside of the shell is of a rich, yellowish brown 

 color, but it is white within the aperture.. The 

 lines of growth are very distinct, and mark the shell 

 in a pleasing manner. Its breadth is from half an 

 inch to an inch. 



Helisoma bicarinatus, Say, bi-car-i-na'-tus, resem- 

 bles the last but is much smaller. Whorls with a 

 sharp angle or keel, both above and below the suture. 

 The specimens before me were collected in Portland, 

 Oregon. 



Xow we come to a fresh-water mollusk that is very 

 widely spread, and many of its varieties have 



