ACEPHALES. 109 



of the order, and make its leading characters at once familiar 

 to you. The structure of the shell and of its hinge, and the 

 habits of the animals, will be afterwards detailed ; all I have 

 at present to do is to indicate the names of the great families 

 among them. First on the roll we find the Ostraces or 

 oysters, whose mantle is entirely open in all its circumfer- 

 ence, and the valves of the shell are closed by a single cen- 

 tral muscle. The edible oyster is the true representative of 

 the family, but Cuvier includes in it also the Pectens, the 

 Anomiae, the Spondyli, the Malleus or hammer-shell, the 

 Avicula or pearl-oyster, the silk-spinning Pinna, the Area, 

 and a considerable number of fossil genera allied to one or 

 other of those just mentioned. The second family is the 

 Mytilaces or mussel-tribe, in which the mantle is open in 

 front, but with a distinct aperture for the excrementitial 

 discharges, and there are two adductor muscles to close the 

 shell; one of them, however, is very small in some of the 

 genera, and placed near the hinge. The family embraces 

 the sea-mussels as well as those of our rivers and ponds, and 

 some (Lithodomus, Coralliophages,) which have the faculty 

 of boring into solid substances. The Camacees have the 

 mantle close, perforated with three apertures ; through one 

 the foot is protruded, the next serves for the ingress and 

 egress of the water necessary to respiration, and the third 

 for the discharge of the excrements. The gigantic Chama 

 is the best known of the family, which is a small one. It 

 conducts us to the Cardiaces, so called from the resemblance 

 of the shell to a heart ; they have two adductor muscles, a 

 mantle open in front, and furnished besides with two sepa- 

 rate tubular apertures at one extremity. The cockle (Car- 

 dium) is of this family, which is widely spread and abundant 

 in variety, for the genera Donax, Cyclas inhabitants of 

 fresh waters Tellina, Venus, and Mactra, the most numer- 

 ous in species of all bivalves, with all the kinds allied to 

 these, are reputed to belong to it. The fifth family is named 

 Enfermes, because the mantle is open only at the anterior end 

 or near the middle for the passage of the foot ; and at the 

 other end there are two tubes, often bound in one common 

 envelope, and capable of being extended to a considerable 

 length. The shell gapes more or less ; that is, the animal 

 cannot by any effort so aptly close the valves, but that an 

 open space, generally at each extremity, remains. The En- 

 fermes burrow in the sand to considerable depths, where you 

 may dig up at low tides the Myse and the Solens, or razor- 

 shells, the best specimens of their family. Cuvier, however, 

 places in it also the Pholas, and the " fell Teredo " of which 



