ETHERIA. CONCHIFERA. 87 



and inserted towards the lateral extremities of the valves. These muscles always 

 leave in the interior of the shell impressions more or less marked. The order is 

 divided by Lamarck into four sections, viz. 1. CAMACEA, or ambiguous Conchifera ; 

 2. LAMELLIPEDES 3. TENUIPEDES; 4. CRASSIPEDES; and fourteen families, 

 as follows : 



1. Shell irregular, always inequivalve. Camacea. 



2. Shell regular, generally equivalve. 



a. Shell close at the lateral extremities when the valves are shut. 

 LAMELLIPEDES. Foot flatten ed, lamelliform, not posterior Nayades, Trigonacea, 



Arcacea, Cardiacea, Conchacea. 



b. Shell in general gaping at the lateral extremities when the valves are closed. 

 TENUIPEDES. Mantle scarcely united before : foot small, compressed ; the gape of 



the shell not very large. 



Ligament external. Nymphacea, Lithophagi. 



An internal ligament, with or without complication of the external one Cor- 

 Mactracea. 



** CRASSIPEDES. Mantle with the two lobes united before, either partially or 

 wholly ; foot thick, posterior ; gape of the shell always considerable. Myarice, 

 Soknides, Pholadaria, Tullcola. 



SECTION I. 



Shell irregular, always inequivalve. 

 FAMILY I. CAMACEA. 



Shell inequivalve, irregular, fixed ; one large tooth or more at 

 the hinge ; two separate, lateral muscular impressions. 



The shells of this family have the ligament exterior, and sometimes sunk irregu- 

 larly inwards. In the structure of the hinge they have some analogy with the Tri- 

 dacnites. They are often lamellar or rough with asperities, and their beaks are al- 

 ways unequal, often large and bent inwards. The animal has short separate syphons. 

 The shells of this group are generally fixed upon rocks, corals, and often upon one ano- 

 ther. 



Gen. 1. ETHERIA, Lam. 



Shell irregular, inequivalve, adhering ; beaks short, and appear- 

 ing as if sunk in the base of the valves ; hinge without teeth, 

 waved, subsinuated, unequal ; two distant lateral oblong mus- 

 cular impressions ; ligament exterior, tortuous, penetrating in 

 part into the shell. 



The shells of this genus, from being fixed to rocks in deep water, were formerly 

 but imperfectly known. Their shell is lamellar or foliaceous, like that of the oyster, 

 and fixed to other bodies by the lower valve. 



E. elliptica, Lam. Shell elliptical, flattened, dentated towards the 

 apex ; an oblong callosity on the base of the shell. Inhabits In- 

 dian seas. An. Mus. x. pi. 29 and 30, fig. 1. 



E. semilunata, Lam. Shell obliquely oval, rounded, gibbous ; beaks 

 approximated, subequal; posterior margin straight. Inhabits rocks 

 on the coast of Madagascar An. Mus. x. pi. 32, tig. 1, 2. 



Gen. 2. CHAMA, Lam. Lin. 



Shell irregular, inequivalve, fixed ; beaks bent, unequal ; hinge 

 with a single thick oblique subcrenulated tooth, entering into 



