404 THE ZOOLOGIST. 



the "step" or shelf in question is shown from below in the 

 6mpty shell. In fig. 12 the upper surface has been partly 

 removed, and the mantle-covered dorsum of the animal exposed. 

 In fig. 11, on the contrary, the sole of the fleshy sucker-foot 

 disk, &c., are demonstrated. Again, fig. 13 illustrates a mid- 

 longitudinal section of the shell and position of the shelf, while 

 fig. 14 gives a view of the same with the viscera, &c., in situ. 

 Fig. 15 is a transverse median section with the soft parts in 

 place, the thin shelf seen separating the foot from the upper 

 soft parts. 



Taking into consideration that the sea-margins of the United 

 States are, so to say, the headquarters of several species of 

 Grepidula, and particularly that of C.fornicata, it behoves us to 

 hear what the American Naturalists, Fishery Commissioners, and 

 others report concerning the structure, habits, and influences 

 of the said mollusc to its surroundings. Leaving aside the 

 works of pure conchologists of both earlier and more recent 

 dates (T. Say and A. A. Gould, or W. H. Dall, for example), 

 we may first quote Theodore Lyman's* simple description of 

 the C.fornicata. He remarks that they are: — '* In great num- 

 bers on odd shells, generally those occupied by Eujjagurns 

 [Hermit Crab] , the smaller ones often sticking to the larger. 

 The expanded animal has two snail-like horns with eye-specks 

 at their bases, and between them a pair of lobes (tentacles?). 

 The front part of the body and head have the form of a rather 

 thin plate of tissue, which is very movable, while the sucking 

 disk rests on the 'step' in the shell, and is very thick and 

 muscular ; the front edge of the disk is prolonged in the shape 

 of a movable flap. Round the edge of the shell runs the 

 mantle, which may be considerably contracted. The gills lie in 

 a sheet on the inside of the roof of the shell. The Crepididcs 

 were laying eggs which adhered in clusters to the surface on 

 which the animals lay. Those clusters contained a bunch of 

 transparent sacs, each of which was full of embryos. These 

 embryos, before leaving the sac, have an active motion. They 

 take on the form of little bags, tied as it were near the top ; the 

 bag itself is filled with yolk-cells, while the loose flaps above the 



* " Habits of Animals observed at West Yarmouth, Mass.," in Proc. 

 Boston Soc. Nat. Hist. vol. vii. p. 78 (1859-61). 



