CERITHOPSTS. 181 



half thickened, so as to form eye-pedestals, the eyes being 

 placed near the middle of the tentacles ; the foot is short 

 and broad, rounded at the back, and squarish in front, a 

 little contracted at the sides. It bears an operculum, 

 which, as to construction, is called concentric, although the 

 nucleus of the successive layers is lateral. 



II. Cerithopsis tuhercidaris is a very curiously-con- 

 structed animal, having a shell so much like that of a Cerl- 

 tlmmi as to be, in itself, quite undistinguishable. Its oper- 

 culum is placed on a lobe, or flap, at the back of the foot, 

 which is grooved underneath, the groove terminating in the 

 centre by a perforation. I should not be at all surprised 

 to hear, when the animal is better known, that this groove 

 had something to do with a byssus, and that this mollusc, 

 like Rissoa j^ai'va and Centliium, ohtusum, was in the habit 

 of suspending itself by byssal cords during the hybernat- 

 ing season. This notion however, it must be confessed, is 

 rather loosely hazarded, and is not at all founded upon any 

 actual examination of the structure. 



The tentacles are blunt, thickened at their bases, with eyes 

 on the thickened part, near together; and in front of the 

 head, above the fore part of the foot, is a flap, or mentum, 

 resembling that which, in Natica, is reflected so as partly 



