346 Dentaliid^. MOLLUSCA. Patellid^. 
forated by a hole great]}' resembling that of an ordinary 
key-hole — hence the name of “Key-hole Limpets.” — 
See fig. 2‘25. 
Family — DENTALIID^ {Tooth-shells). 
The Tooth-shells are very different in external ap- 
pearance from the Fiss^irelUdce ; they are elongately 
conical, tubular, curved, open at each end, and at- 
tenuated posteriorly. Notwithstanding this apparent 
difference of structure, the most usual way of regarding 
the Tooth-shells “has been to considerthem as elongated 
Fissurellre, with the eyes and tentacles wanting, and 
the foot rudimentary and tubular ; the perforated small 
end of the shell corresponding with tlie orifice of the 
vertex of Fissurella and subserving the same purpose.” 
—{Aclam>.^. The body of the animal is formed like the 
shell, tapering gradually to the posterior extremity, and 
is attached to the shell near its posterior orifice. The 
stomach is furnished with a strong gizzard, and they are 
decidedly carnivorous animals. They feed upon Fora- 
minifera and small bivalve shells, and are usually found 
buried in the sand, or sandy mud, in from ten to one 
hundred fathoms. About thirty species are described, 
and they are found in India and the West Indies, in the 
Mediterranean, on the coasts of Norway, and the shores 
of Great Britain. 
Genus Dentai.ium. — T his genus has a tubular, 
tapering, and curved shell, with a smooth or longitudi- 
nally striated surface, and a circular and entire aperture. 
Genus Entalis. — This genus is distinguished by 
merely having the circular aperture longitudinally fis- 
sured on the dorsal or hinder margin. 
Family— TECTURIDtE {False Limpets). 
In external appearance, the shells of this family bear 
so close a resemblance to the rock limpets {PatelUdm) 
that it is exceedingly difficult to distinguish them. Of 
almost all the genera of Mollusca that present a simi- 
larity of appearance, these two are probably, says Dr. 
Gray, the most remarkable examples, “ on account of 
the extreme dissimilarity of their animals, which are 
referable to two very ditlerent orders ; while the shells 
are so perfectly alike, that, after along-continued study 
of numerous species of each genus, I cannot find any 
character by which they can be distinguished with any 
degree of certainty. The agreement in the internal 
structure of their shells is equally complete ; yet the 
animal of Patella has the branchise in the form of a 
series of small plates disposed in a circle round the inner 
edge of the mantle, while that of Lottia {Techira) has 
a triangular pectinated gill, seated in a proper cavity 
formed over the back of the neck, within the mantle. 
This difference in the respiratory organs of animals 
iidiabiting shells so strikingly similar,” continues Dr. 
Gray, “ is the more anomalous, inasmuch as those 
organs commonly exercise great influence on the general 
form of shells — a circumstance readily accounted for 
when we reflect that a principal object of the shell is to 
afford protection to those delicate and highly-irnportant 
parts.” The Tecturidee have a large foot, but no 
operculum. The shells are depressed, conical, or cao- 
shaped ; the aperture is large, and generally of a 
beautifully porcellanous appearance internally. The 
species are probably numerous ; they are littoral in their 
habits, and are usually found between tide marks feed- 
ing on the sea-weed. “ Their locomotion is very 
limited, and in their geographical distribution they are 
found both in temperate and tropical countries.” 
— {Adams.) 
Family — PATELLIDiE {Rock Limpets). 
We have already briefly described the difference in 
the structure of the respiratory organs between the 
Rock limpets and the False limpets, and mentioned the 
striking resemblance in the shells of the two families. 
The Rock limpets have a short muzzle, subulate ten- 
tacles, and fringes on the margin of the mantle. The 
shell is simple, conical, with a sub-central apex and a 
wide aperture, smooth within, but not of so rich a por- 
cellanous appearance as in the False limpets. The 
species of Patellidcs are very numerous, upwards of one 
hundred having been described. They are strictly 
littoral in their habits, and are world-wide in their 
distribution. 
Genus Patella. — By far the greater number of 
species belonging to the family are contained in this 
genus. These shells are usually found fixed upon rocks 
on the shore; hence their name of Rock limpets. Some 
of the species on the coasts of South America and 
California attain a large size. Mr. Cuming informs 
the writer, that the species known as the Patella rnexi- 
cana is often used by the natives as a wash-hand basin ! 
The Patellce possess the power, in a certain degree, 
of excavating holes in the rocks and other bodies upon 
which they live. 
I THE SPOON LIMPET OF THE CAPE {Patella cochlear) 
lives almost exclusively attached to a large species of 
the same genus, and on the surface of this it forms a 
flat disk, exactly the size of its mouth. To form this 
disk and increase its size, it has been observed to absorb 
any coralline or other similar substance with which the 
larger shells are abundantly covered. Some of the 
species of the preceding family of False limpets possess 
the same power ; as for instance the Tectura parasitica, 
which obtains its specific name from its habit of living 
upon other shells, and upon the surface of which it 
forms a hollow answering to the size and shape of the 
rim of its shell. But in no species has this been so well 
observed as in the Common limpet of our own coasts. 
THE PATELLA VULGATA is in the habit of hollowing 
out a space in the site upon which it has settled, and 
which varies in depth according to the softness or hard- 
ness of the rock upon which it has taken up its abode. 
In the soft calcareous rocks of the Isle of Thanet, the 
pits so made are half an inch deep ; but upon hard 
limestones the depressions are much shallower, and it 
is in the case of adults only that we find the rock worn 
down, the margin of the shell often accommodating 
itself to the inequalities of the surrounding surface. 
The limpets adhere very firmly to the rocks upon which 
they are located, and it is often very difficult to remove 
them. Reaumur ascertained that a weight of twenty- 
eight or thirty pounds was required to overcome their 
