GASTEROPODA. 979 
EHzample, P. longicostata. Pl. XI., Fig. 22. 
Shell oval, with a sub-central apex; surface smooth, or 
ornamented with radiating striz or ribs; 
margin eyen or spiny ; interior smooth. 
Animal with a continuous series of bran- J--5,! 
chial lamelle; mantle-margin fringed ; eyes 
sessile, externally, on the swollen bases of 
the tentacles; mouth notched below. Lingual A) 
teeth 6, of which 4 are central, and 2 lateral; =gey 
uncini 3. Fig. 120 shows the teeth, but not ///' 
the uncini of P. vulgata. The Cape limpets 
(e.g. P. denticulata) have a minute central 
tooth, which is wanting in any other species 
: : : : Fig, 120. Patella 
hitherto examined. (Wilton.) vulgata. 
The dental canal of the common British (0"8!*) Wilton.) 
limpet (P. vulgata) is rather longer than its shell; it has 160 
rows of teeth, with 12 teeth in each row, or 1,920 in all. 
(Forbes.) The limpets live on rocky coasts, between tide- 
marks, and are consequently left dry twice every day; they 
adhere very firmly by atmospheric pressure (15 lbs. per square 
inch), and the difficulty of detaching them is increased by the 
form of the shell. On soft calcareous rocks, like the chalk of 
the coast of Thanet, they live in pits half an inch deep, pro- 
bably formed by the carbonic acid disengaged in respiration ; on 
hard limestones only the aged specimens are found to have worn 
the rock beneath, and the margin of their shell is often accom- 
modated to the inequalities of the surrounding surface. These 
circumstances would seem to imply that the lmpets are 
sedentary, and liye on the sea-weed within reach of their 
tongues, or else that they return to the same spot to roost. On 
the coast of Northumberland we have seen them sheltering 
themselves in the crevices of rocks, whose broad surfaces, over- 
grown with nullipores, were covered with irregular tracks, 
apparently rasped by the limpets in their between tides 
excursions. * 
The limpet is mucn used by fishermen for bait; on the coast 
of Berwickshire nearly 12,000,000 have been collected yearly, 
until their numbers are so decreased that collecting them has 
become tedious. (Dr. Johnston.) In the north of Ireland 
they are used for human food, especially in seasons of scarcity; 
* If limpets are placed in stale water, or little pools exposed to the hot sun, they 
creep Out more quickly than one would expect; the tracks they leave are very 
peculiar, and not likely to be mistaken when once seen, 
