— 9 



Fio. 4. Mania macrosch.isuia. Two-thirds natural size. (A) Interior 

 upper or left valve. (B) Interior lower or riglit valve. 



Left valve with four muscle scars (one atlductor. three byssal muscles) ; sliell 

 very delicate, smooth te.xtured and usually translucent, rarely taken 



between tides. 



Auoniia peruviana 



p. 26; pi. 



fif 



bb. Valves similar or unlike, without byssal notch. 



c. Shell irregular, without radiating ribs or lateral projections on either side of 

 umbo, permanently attached by left valve ; mantle without eyes ; flesh 

 white or tinged M'ith greenish ; found in colonies or masses. 



Oysters Ostrea P- 24 



cc. Shell with radiating ribs and lateral projections or "ears"' on either side of the 

 umbo ; regular in shape and free for at least part of adult life ; margin 

 of mantle with numerous minute eyes; flesh tinged with orange or red; 

 found separate and free during early adult life at least. 



Scallops 



d. F'ree and regular in shape for entire life, interior of hinge area not purplish; 

 shells light; capable of .swimming actively. 



Pcctcn circiilaris 



p. 24 ; 111. 1 , figs. 1, 2 



dd. Free and regular for part only of adult life, later attached l)y right valve, 

 which retains pattern of young free shell ; interior of hinge area 

 purplish ; shells heavy and thick. 



H in iviti's gifjan tens 



p. 2.5 ; pi. 1 , fig. 3 ; pi. 2 , figs. 1, 2 



aa. Two adductor mus(4es present; adult eitlier attaclied by flexible Ijyssiis, moving 

 by means of muscular foot or burrowing, usually not lying on one side, never 

 attached by shell (in forms here treated), nor swimming (except Solen). 

 (See fig. 3) 

 b. Posterior adductor large, anterior small, umbo near anterior end of shell ; 

 mantle edges fused at only one point, forming an exhalent siphonal aper- 

 ture ; no pallial sinus ; adults attached to firm objects, or anchoi-ed in mud, 

 by byssus, never lying on one side ; never moving when advilt ; shells brown 

 or shiny black. 



Mussels 



c. Umbo at extreme (attached) end; shells not bearded; shiny black, usually 

 found in "beds" attached to rock or the like. 



Mi/tilus 



p. 26 ; pi. 3 , fig. 4 ; pi. 4 , figs. 1, 2 



