'Tusk-shells, Mail-shells, and Limpets 195 



above. The beak remains spiral as in the embryo. 

 The " Keyhole " of the previous species has a 

 thickened margin internally, but in the 

 present species there is a vaulted process 

 beneath the slit. The shell is white, 

 ribbed in a manner similar to that of 

 F. (jnvca, but less rugged. It is a deep- 

 water (25 to 90 fathoms) species, and a 

 northern — not being found south of Yorkshire. 



Common Slit - limpet (Emarginida fissara). In 

 the genus Emdrginida the slit instead of appearing 

 in very early life and travelling to the crown of the 

 shell, as in Fissurella and Fanctiirella, 

 appears when the animal is nearly 

 adult and remains at the margin, becom- 

 ing partially filled up with age. The 

 mantle protrudes from this slit to form 

 an excurrent siphon, as in the other 

 members of this family. The shell is white, and 

 crossed by ribs much as in the other species, but here 

 they are less numerous. It is generally distributed, 

 and may be found on rocks and stones from extreme 

 low water to a depth of over 100 fathoms. The 

 Rosy Slit-limpet (E. rosea) is a much smaller species, 

 with proportionately narrower shell of distinct 

 Phrygian cap forju. The ribs are more ecjual 

 in thickness, and the pits caused by their 

 intersection round, wliereas in the preceding 

 species they are more square. The slit is 

 shorter, the mouth often tinted with bright rose 

 colour, and the usually white shell sometimes tinged 

 with pink. It is a southern form, but may be 

 found commonly in deep water (7 to 25 fathoms) 



