TOOTH SHELLS. 219 



which he displays in undermining these far exceeds the rapidity of the most practised oyster- 

 opener at a London fishmonger's shop. 



On the western coast of South America there is a Limpet which attains the diameter 

 of a foot, and is used by the natives as a basin. 



FAMILY XXV. DENTALIADJ3. 



Genus Dentalium, " Tooth-shells." The shell is like a curved tube, open at each end, gradually 

 increasing in size from the posterior to the anterior end ; the surface is either smooth or ribbed longi- 

 tudinally ; the mouth is round (not contracted at the aperture, like the genus Ditrupa, which is an 

 annelid). The animal is attached to its shell near the smaller end; the head is rudimentary, the eyes 

 and tentacles are wanting, the mouth is fringed, the foot is pointed. The Dentalia are all animal 

 feeders, devouring foraminifera and minute bivalves ; they bury themselves in the mud, and range 

 in depth from ten to 100 fathoms. Their distribution is nearly cosmopolitan. Thirty 

 Jiving species are known. 



In the Cavern of Bruniquel, Valley of the Aveyron, Department of Tarn-et-Garonne, 

 several shells have been found, evidently collected and used by the pre-historic occupants 

 as ornaments. These included Dentalium, Natica, Nassa, Pectunculus, Scalaria, Valuta, and 

 Gypreeet, several of which had been perforated. 



Corresponding to the Cowry currency of Asia and Africa is the American loqua, or 

 Dentaliwm, a shell found chiefly at the entrance of the Strait of De Fuca, and employed 

 both for ornament and money. The Chinooks and other Indians of the Northern Pacific coast 

 wear long strings of loqua shells as necklaces and fringes to their robes. These have a DENTALITJM 

 value assigned to them, increasing in proportion to their size, which varies from about an inch TINUM AN 

 and a half to upwards of two inches in length. Mr. Paul Kane writes : " A great trade 



is carried on among all the tribes in the neighbourhood of Vancouver Island 

 through the medium of these shells. Forty shells of the standard size, extending a 

 fathom's length, are equal in value to a beaver's skin ; but if shells can be found 

 so far in excess of the ordinary standard that thirty-nine are long enough to make 

 a fathom, they are worth two beavers' skins ; and so on, increasing in value one 

 beaver skin for every shell less than the first number." 



FAMILY XXVL CHITOXmaE. 



Genus Chiton* (Linnaeus). Unlike the other Mollusca already described, the 

 shell of Chiton is made up of eight imbricated plates, fixed transversely on the 

 CHITOX MAGNIFICUS. back of tjie anmia l, which enable it, when caught, to roll itself up like a Wood- 

 louse or an Armadillo ; the border of the mantle is bare, or covered with minute, 

 plates, hairs, or spines. Like the Limpets, the Chitons have a broad creeping disc; they have 

 long series of lingual teeth, but 110 eyes or tentacles. More than two hundred species occur, living 

 all over the world, from low water to a hundred fathoms. 



ORDER II. PULMONIFERA.t 



In this order are placed all the air-breathing Snails. Many of them have forms externally 

 similar to the Sea Snails, whose tribes we have already enumerated ; but they differ in this essential 

 character : that whereas the BRANCHIFERA carry on their respii-ation in water by gills or membranes 

 (like fishes), and, as a rule, lead an aquatic existence, the PULMONIFERA admit air into their breathing 

 chamber, which is lined with finely-branching vessels, and is, in fact, a simple form of lung ; and they 

 mostly lead a terrestrial existence. 



In the Pulmonifera, or Air-breathing Snails, provision is made for the admission of air directly 

 into a respiratory chamber (m) formed by the mantle. In some of the Slugs this is placed at the 

 hinder extremity of the body, as in Testacrlla, in the others at one side of the mantle; and in the 

 Garden Snail the aperture is nearly in front, beneath the mouth of the shell (ri). The mouth (a) has a 



* Greek, chiton, a coat of mail. f Latin, pulmo, a lung ; and/era, I bear or carry. 



