CONCHOLOGY. 85 



Shell. Thin, inequivalve, regular, subtrigonal ; one of the 

 valves larger and more dilated than the other, which is sometimes 

 operculiform ; hinge condyloid, in a straight line, and formed by 

 two oblique articular surfaces in one valve placed between corres- 

 ponding projections in the other. Inhabits the sea at the Zetland 

 Islands. Two species are found in America. Thirteen living 

 species. Thirty-eight fossil. 



Terebratula dilatata. Terebratula globosa. 



T. flavescens. T. caput-serpentis. 



T. dorsata. T. psittacea. 



T. Vitrea. T. rotunda. 



T. dentata. T. pisa. 



T. sanguinea. 



T. septentrionalis.* T. truncata.* 



3. Genus Livgida, PI. IX. 



Animal. Depressed, oval, somewhat elongated, inclosed be- 

 tween two lobes of a mantle, slit throughout its anterior half, and 

 having pectinated branchiae adhering to the internal surface ; 

 mouth simple, having on each side a long tentacular appendage 

 ciliated in all its external edge, and rolling itself up spirally in the 

 shell. 



Shell. With an epidermis, subequivalve, equilateral, depressed, 

 elongated, truncated anteriorly ; the summit median and posterior ; 

 no trace of ligament ; a long fibro-gelatinous peduncle fixing the 

 shell vertically to submarine bodies ; muscular impression multiple. 

 Inhabits the Indian Ocean. One species. 

 Lingula anatina. 



CLASS IV. 



M O L L U S C A . 



Animals soft, inarticulated, furnished with an anterior head^ 

 projecting or salient ; most frequently with eyes and tentaculse, or 



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