MYTILIDjE. 511 



forming a single, posterior muscular impression. In the 

 young and free state shell equivalve, shaped like A/todon, 

 with two distinct muscular scars, and an external, con- 

 spicuous, elongated ligament. 



Si/n. Acostoea, D'Orbigny. 



Ex. M. lobata, Ferussac, pi. 120, fig, 4, 4, a, 4, b. 



The Acostaa Guaduasana, from the river Magdalena, 

 near Bogota, is the same as the Miilleria lobata described 

 by Ferussac and Sowerby, and presents a curious example 

 of being dimyary and locomotive when young, and mono- 

 myary and fixed when in the adult condition. 



Fam. MYTILLD^. 



Labial palps elongate, pointed, free. Mantle-margins free, 

 or united behind to form a more or less complete anal tube ; 

 gills two on each side, nearly equal, elongated, dependent, 

 united behind to each other and to the mantle. Foot 

 narrow, strap-shaped, furnished with a byssal groove. 



Shell elongated, equivalve, closed ; surface of valves 

 covered with a dark, thick epidermis, more or less pearly 

 within. Hinge edentulous, or sub-dentate ; ligament sub- 

 internal, marginal, linear. Anterior muscular impression 

 small and narrow, the hinder large and obscure. 



Marine, byssiferous. 



Sub -fam. MYTILINiR. 



Hinder part of mantle only slightly produced; anterior 

 adductor muscle small. 



