LECTURES ON MOLLUSCA. 219 



form, described as Gadinia pente-goniostoma has been found with six, 

 five, four, three, two, corners, or only one ; or quite round, which is its 

 normal state. So much may we err by describing from single speci- 

 mens. 



Family Kcmmidm. {False Limpets.) 



The shells of all the Limpets are so like each other that no charac- 

 ters have yet been found to distinguished them generic-ally. But the 

 accurate Russian naturalist Eschscholtz,. when examining the Limpets 

 of the Californian coast, found that they differed materially from the 

 true Limpets in the shape of the gill. While the ordinary Rock-lim- 

 pets have the gill greatly developed, going all round the margin of 

 the shell, as in the oysters, these deeper water species have one small 

 gill on the left side of the neck, like the Top-shells. The teeth also 

 are in rows of not more than six each. It would have been very con- 

 venient if these very different gills had left their different marks inside 

 the shells ; but all the fancied marks turn out fallacious ; the animals 

 of reputed Acmceas turning out to be Limpets, and vice versa. Further, 

 among the single-gilled Limpets, there are now found considerable 

 differences ; the large Tecturina grandis of the Californian coast being 

 the type of a separate group. The white, conical Scurria mitra, which 

 makes holes for itself in the roots of sea weeds in the west temperate 

 regions of both North, and South America, (avoiding the intermediate 

 tropical region,) has a fringed mantle, looking like a gill, all round 

 the inner edge of the shell. The shells of the beautiful group Scutel- 

 lina are thin, finely sculptured, and very glossy inside. They often 

 have a rudimentary pillar lip, like Navicella, which caused the west 

 American species to be described by Prof. C. B. Adams as a Crepidida. 

 The little Scotch Pilidium has a somewhat similar shell. The animal 

 of the boreal Lepeta is blind ; its teeth are curiously ornamented like 

 a stag's head. 



Family Patellid^i. (True Limpets.) 



The largest known Limpet (Patella mexicana) inhabits the rocks of 

 west tropical America, growing to be a foot across, and of capacity 

 large enough for a French lady's wash hand basin; else, this tribe, so 

 abundant elsewhere, is remarkably absent from North America. The 

 rocky shores of the Old World are covered with them, almost always 

 above the region of the Acmauds ; sometimes at such high levels that 

 they can rarely be dashed over with sea water or find anything to eat. 

 Like the Ear-shells, they adhere very firmly to the rocks when once 

 touched, by means of their strong muscular foot, grooved across the 

 middle. The tongue of the common English Limpet is longer than 

 the shell itself; containing 160 rows of twelve teeth each, or 1,920 little 

 glassy hooks. With these it rasps the nullipore and sea-weed, prin- 

 cipally in the night. It has the organs both of adhesiveness and in- 

 habitiveness large, growing according to the shape of the rock which 

 it. selected, and where it always returns to roost. In one county of 

 Scotland twelve millions have been collected in a year for bait ; and 

 near Larme, in Ireland, many tons' weight are annually collected for 



