GENERA OF SHELLS. 43 



byssus. They usually gape a little at the middle of 

 the contracted margin of the posterior side. 



M. papuana, tulipa, albicosta, Guyanensis, Adriatica, 

 pulex, vagina, picia, sulcata, plicatula, semifusca, securis, 

 purpurata, barbata, discrepans, discors, trapesina, cinna- 

 monea, siliciiia, plicata, semen, hdiophaga, caudigera, — 

 Americana, castanea. 



Mytilus. Shell longitudinal, cquivalve, 

 regular, pointed at base, fixed by a byssus ; 

 beaks nearly straight, terminal, pointed ; hinge 

 lateral, generally toothless ; ligament marginal, 

 subinterior ; muscular impression elongated, 

 clavate, sublateral. 



The mytili are all sea shells, and are not foliated, nor 

 gaping at the superior margin, in which they differ 

 from the pinna, which they otherwise resemble. 



(a.) Shell icith longitudinal furroivs. 



M. raagellanicus, erosus, crenatus, decussatns, hirsutns, 

 exustus, bilocularis, ovalis, uslulatus, Domingensis, Sene- 

 galensis, — hamatus, lateralis. 



(b.) Shell loithout longitudinal furrows. 



M. elongatus, latus, zonarius, canalis, ungulatus, viola- 

 ceus, opaius, smaragdinus, perna, afer, achatinus, unguîaris, 

 planulatus, borealis, augustanus, corneus, Galloprovincialis, 

 edulis, abbreviatus, retusus, hesperianus, incurvatus, line- 

 5lus, lacunatus, — cubitus, leucophaBatus. 



Pinna. Shell longitudinal, wedge shaped, 

 equivalve, gaping at summit, pointed at base ; 

 beaks straight. Hinge lateral, toothless ; lig^ 

 ament marginal, linear, very long, subin-r 

 terior. 



