54 UNIVALVES. 



The ear-shaped shells with an unperforated lip may be divided into 

 Whorls twoOtina. 



Whorls more than two 



Very thin and transparent 



First whorl twisted Lamellaria. 



Rather thin and opaque 



First whorl not twisted Velutina. 



Otina is very frail, and about a tenth of an inch in height and 

 diameter, being in shape not unlike a cap of liberty. The others 

 are much longer, Lamellaria measuring almost three-quarters of an 

 inch, and Velutina, of which there are two species, averaging half 

 an inch or more. 



There are thirteen genera whose shells can fairly be classed as 

 cap-shaped, like that of the limpet. Two of these are oblong in 

 shape, Ancylus and Testdcella, the first being the fresh-water limpet, 

 the other the carnivorous slug which carries the shell jauntily on 

 the back of his tail. This shell is ridiculously small for the size of 

 the animal, and, like those of all the ^slugs, is the representative of 

 something much larger in the past. In Ancylus the crown is in the 

 middle, in Testacella it is on the right-hand side, and the mouth of 

 Ancylus is quite open, while that of the other is folded under along 

 the side beneath the crown ; in fact, the differences are strongly 

 marked, though our sorting has brought the two genera together. 



Two of the group have a curled and twisted beak. These are 

 Crepidula and Capulus, the first of which has a septum across the 

 mouth, the other having the mouth free. Both shells are somewhat 

 oblong, but Crepidula is longer in the line of the beak, while Capulus 

 is longer across that line. Capulus is a stout, sturdy, independent 

 sort of a shell ; Crepidula, the slipper limpet, is a parasite, and found 

 its way into the British list from having been found on the American 

 oysters transplanted to this country, and a specimen on the oyster 

 is in the London Natural History Museum. 



Another group of three genera can be recognised as being slit. 

 Fissurella has the slit at the crown, Emarginula has the slit on the 

 margin, and Puncturella has the slit between margin and crown. In 

 Puncturella the beak is twisted to the left, and the slit so covered by 

 an internal sheath that it is not at first very clear ; in Emarginula 

 the slit is apparent at once, as it makes a. clear cut in the edge of the 

 shell, and extends inwards for an appreciable distance. 



Two genera have the crown near the front margin. Of these, 

 Tectura is much depressed, while Helcion is almost as high as it is 

 long. Tectura is like the roof of a house, Helcion like a lady's 

 bonnet ; one coarse and weather-beaten, the other bright and glossy, 

 and beautifully streaked with narrow radiations of blue. In a group 

 by itself, with the crown almost central, but not quite, we can put 

 Patella, the common limpet, which is unmistakable in its many 

 varieties. Looking at the margin of Patella you can tell whether his 

 home is hard or soft. If the rock be hard, he modifies his margin to 

 suit it ; if it be soft, he scoops it out into a pit so deep that little 

 more than the crown appears above it, and the crown this is the 

 important point is never quite in the centre. 



