Monograph of the LameUihrancJiiata. 215 



opeti throughout, not forming siphons ; the pallial impression always 

 simple and entire ; two or more muscular impressions in each valve. 



The order contains the following families: 1. Limidce ; 2. 3Iytilida: 

 3. Arcadce ; 4. NucuUdce ; 5. Trigoniidce ; 6. Unionidce ; 7. Lucinidce. ; 

 8. CycIasidcB ; 9. Cyprinidai; 10. Astartidce ; 11. Tridacnidcc ; 12. Car- 

 diidce, etc. 



The fliuiily Mytilidce is described as follows : Shell elongate, oval, 

 narrow in front, "widening posteriorly, more or less closed (equivalve, 

 pallial impression entire), cartilage very long, marginal or nearly so, 

 supported by an internal plate nearly parallel with the hinge line ; 

 beaks close to the anterior end or terminal ; two or three muscular im- 

 pressions in each valve. Animal: mantle more or less open; gills 

 fringed or entire ; mouth with appendages ; foot narrow, tongue- 

 shaped, with a fibrous bygsus at its base, to attach the shell to foreign 

 bodies, having a small sinus in the anterior part of the ventral margin, 

 from which an oblique, impressed furrow extends toward thq beak, de- 

 fining the anterior lobe. 



The genera Modiolopm, Cyp'ieardltes, etc., belong to this family. 



The family Trigonidcc is described as follows : Shell equivalve, close, 

 trigonal, with the umbones directed posteriorly ; ligament external ; 

 interior nacreous ; hinge teeth few, diverging ; pallial line simple. 



The genus Lyrodesma belongs to this family. 



Genus Avicida, Klein — (Bruguire). 



[Etym. — Avicula, a little bird.] 



Shell obliquely oval, very inequivalve ; right valve with a byssal 

 sinus beneath the anterior ear ; cartilage pit single, oblique ; hinge 

 with one or tw'O small cardinal teeth, and an elongated posterior tooth, 

 often obsolete; posterior muscular impression (adductor and pedal) 

 large, sub-central ; anterior (pedal scar) small, umbonal. 



Avicula demissa — (Conrad, 1842). 



Obliquely subovate, compressed, extended posteriorly into a broad, 

 triangular wing ; anterior wing short, obtuse ; surface marked by close, 

 sharp, imbricating, lamellose strias ; posterior wing extending beyond 

 the line of the posterior extremity of the shell ; anterior and posterior 

 margins nearly parallel, and but slightly obli(jue. 



It is characterized by the sharp elevated concentric striaj, which are 

 crowded together on the posterior wing, and at its junction with the 

 body of the shell. 



