80 CONCHOLOGY. 



3. Genus Lingula. 

 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 poste- 

 rior ; 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 L L U S C A. 



Animals soft, inarticulated, furnished with an anterior head, 

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

 possessing, at their summit, arms disposed in the form of a coro- 

 net : their mouth either short, elongated, or tubular, exertile, and 

 generally armed with hard parts. Mantle diversified, having its 

 edges free on the sides of the body, or the lobes united, forming a 

 sack, which in part envelopes the animal ; gills or respiratory 

 organs various, circulation double, one particular, the other gen- 

 eral; heart unilocular, sometimes with the auricles divided, and 

 very distant; no medullary cord along the body, but scattered 

 nerves and ganglions. Twenty-two families. 



FAMILY I. 



Pteropoka. Six genei-a. 



Some genera of this family are without' a testaceous covering, 

 mentioned only to preserve the family entire. 



