346 



Dentaliid^.- 



-MOLLUSCA.- 



-Patellid.e. 



foratcJ by a hole greatly' resembling that of an ordinary 

 kcy-liolc — hence the name of "Key-hole Limpets." — 

 Sec iig. '225. 



Family— DENTALIID^ [Tooth- shells). 



The Tooth-shells are very different in external ap- 

 pearance from the Fissnrellkhe ; they are elongately 

 L'onical, lubular, curved, open at each end, and at- 

 tenuated posteriorly. Notwithstanding this apparent 

 dilTerence of structnre, the most usual way of regarding 

 the Tooth-shells "has been to considerthem aselongated 

 Fisburella^, with the eyes and tentacles wanting, and 

 the foot rudimentary and tubular ; the perforated small 

 end of the shell corresponding with the orifice of the 

 vertex of Fissurella and subserving the same purpose." 

 — (Ailnmsy 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 

 Jlediterranean, on the coasts of Norway, and the shores 

 of Great Britain. 



Genus Dentalium. — This genus has a tubular, 

 tapering, and curved shell, with a smooth or longitudi- 

 nally striated surface, and a circular and entire aperture. 



Gknus Entalis. — This genus is distinguished by 

 merely having the circular aperture longitudinally fis- 

 sured on the dorsal or hinder margin. 



Family— Tl'XTURIDJE {False Limjieh). 



In external appearance, the shells of this family bear 

 BO close a resemblance to the rock limpets {PatclUdte) 

 that it is exceedingly dillicult to distinguish them. Of 

 almost all the genera of Mollusca that present a simi- 

 larity of appearance, these two are probably, says Dr. 

 Graj-, the most remarkable examples, " on account of 

 the extreme dissimilaritj' of their animals, which are 

 referable to two very difl'ereut orders ; while the shells 

 are so perfectly alike, that, after along-continued study 

 of numerous species of each genus, I eannct find any 

 character by which they can be distinguished withanj' 

 degree of certainty. The agreement in the internal 

 structure of their shells is equally complete ; yet the 

 animal of Patella has the branchias in the form of a 

 series of small plates disposed in a circle round the inner 

 edge of the mantle, while that of Lottia {Tectura) has 

 a triangular pecluiated gill, seated in a proper cavity 

 formed over the iback of the neck, within tlie mantle. 

 This difference iin the respiratory organs of animals 

 inhabiting 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 acco\mted for 

 when we reflect that a principal object of the shell is to 

 atford protection to t^iose delicate and highly-important 

 parts." Tlie Tcvturida have a large foot, but no 

 operculum. The shells are depressed, conical, or cap- 



shaped ; the aperture is large, and generally of a 

 beautifully porcellauous appearance internally. The 

 species are probably numerous ; they are littoral in tlieir 

 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— PATELLID^ {Roek 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- 

 cellauous appearance as in the False limpets. The 

 species of Patcllidcs are very numerous, upwards of one 

 hundred having been described. They are strictly 

 littoral in their habits, and are world-wide in their 

 distribution. 



GiiNUS Patklla. — 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 mexi- 

 cana is often used by the natives as a wash-hand basin ! 



The Patella' possess the power, in a certain degree, 

 of excavating holes in the rocks and other bodies upon 

 which tlioy live. 



THE SPOON LIMPET OF THE CAPE (Pa/t'/fe eochlear) 

 lives almost exclusively attached to a large sijecies of 

 the same genus, and on the surface of this it forms a 

 flat disk, exactly the size of its iimouth. 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 Tectvra 2'arasilica, 

 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 

 ohserved 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 ahode. 

 In the soft calcareous rocks ot tlie Isle of Thanet, the 

 pits so made are half an inch ilecp; 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 roeks upon which 

 they are located, and it is often very ditficult to remove 

 them. Reaumur ascertained that a weight of twenty- 

 eight or thirty pounds was required to overcome their 



