32 Lamarck's Genera of Shells. 



seldom foliated with a slight, usually rather elongated mus- 

 cular impression on each valve. The contraction of the muscle 

 of attachment enables the animal to close the shell completely, 

 (except those which have gaping valves,) but as that, if conti- 

 nual, might be injurious to it, it is provided with an interior, 

 and sometimes double adductor, ligament, first noticed by Dr. 

 Leach, which keeps the valves half open for the free passage of 

 the water, at once counteracting the tendency of the cardinal 

 ligament to open the shell entirely, and relieving the muscle from 

 a state of constant contraction. Most of these shell-fish are 

 fixed to marine bodies by a byssus, and have a tongue shaped, 

 or conical foot, which they use to draw out and attach the fila- 

 ments of the byssus. 



1. Modiola*. 



Shell-subtransverse, equivalve, regular ; posterior side very 

 short. Beaks almost lateral, depressed on the short side. 

 Hinge without teeth, lateral, linear. Ligament cardinal, almost 

 wholly internal, inserted in a marginal channel, beginning under 

 the beaks, and extending to part of the anterior, inferior mar- 

 gin of the valves. One sublateral, muscular impression, elon- 

 gated, axe-shaped. 



Almost all naturalists have hitherto confounded the modiolae 

 with the mytili. They diflTer from them however, in being rather 

 transverse than longitudinal shells, the beaks not being truly 

 terminal, a shght projection of the posterior side extending 

 beyond them ; which projection Lamarck considers as the short 

 side of the shell. Moreover they are rarely fixed by a byssus, 

 although they are spinners, (Jileuses,) like the mytili. Their 

 muscular impression is superficial, and analogous to that of the 

 mytili. They usually gape a little at the middle of the contracted 

 margin of the posterior side. 



Type. Modiola papuana f. 



Shell oblong, solid, whitish violet; anterior side obliquely 

 dilated ; umbones tumid, obtusely angular. 



North America. 23 recent Species, 5 fossil. PI. I. Fig. 78. 



• A little measure, or bucket: diminutive, from wodiw, a iusAf/. t Papuan. 



