LECTURES ON MOLLUSCA. 187 



Nerites, with which they really have scarcely even an external affinity. 

 The creatures are very voracious, armed with a retractile proboscis, 

 and furnished with teeth constructed like those of Cassis and Triton. 

 They have, however, no breathing pipe, the water being conveyed to 

 the gills by a fold in the mantle. The shell consequently has no notch 

 at the pillar, and the operculum (when present) is spiral. 



Family Velutinhle. 



This is a little group of creatures chiefly from the northern seas, 

 with very thin, slightly spiral shells, ending in large round mouths. 

 The mantle of the animal partly covers the shell, as in Ficula. The 

 Velutince live in deep water in the Eastern Atlantic ; Morvillia in the 

 West. In Marsenina the shell is ear-shaped, (as in Lamellar ia ;) and 

 in Onchidiopsis it is simply a horny layer. 



Family Natictme. 



The Naticas are very queer creatures; exceedingly voracious, and 

 yet generally blind ; armed with the usual carnivorous appendage of 

 retractile proboscis and horny jaws; and yet, as they walk, looking 

 more like a lump of fleshy sand than a predacious Gasteropod. Their 

 shells are strong, beautifully formed, and very innocent looking ; 

 having a short spire, hollow pillar, and round mouth. The operculum 

 is slightly spiral, and is generally horny; but sometimes has a shelly 

 coat outside. The great peculiarity of the animal is its enormous foot, 

 which not only envelops the shell, like a mantle, but is doubled up in 

 front so as to form a wedge-shaped digger, with which it plows up the 

 wet sand. The head is hidden behind the plow, and thus protected 

 from the sand ; and as the eyes would be hidden also, they are dis- 

 pensed with. The two largest species of the group are found, one in 

 New England, the other on the Oregon shores. No sooner does the 

 tide go down than they may be seen plowing just below the surface, 

 in the region where bivalves love to hide, a small portion of the shell 

 just protruding over the moving sand. No sooner do they come in 

 contact with an unhappy Tellen, than the plow and the broad foot 

 envelop it, the head stretches out, the trunk is darted out, and the 

 drilling process commences, which ends in the suction of the unfortu- 

 nate bivalve. 



Those who examine the objects on the sea shore in summer time can 

 hardly fail to have noticed some curious sandy, ribbon-shaped, frail 

 substances, curled like a horseshoe. Naturalists have often taken 

 them for zoophytes ; and they have been variously described as Flustra 

 arenosa, Eschara lutosa, Alcyonium arenosum, and Discopora crebrum. 

 It is however nothing but the nest which Mother Natica makes for the 

 protection of her eggs. If held to the light when wet, it will be found 

 to consist of sand, glued together, and filled with little cells arranged 

 in quincunx, each one of which has contained an egg. The Naticas 

 are found in all parts of the world, and have existed in all ages, be- 

 ginning with the palaeozoic. 



In Naiica proper, the operculum has a shelly coat, which is often 



