112 Mr. A. Hancock on the Animal of Chamostrea albida. 



for in dissecting the tube upwards from the anus, I lost it just 

 before it reached the stomach ; I may have been deceived, there- 

 fore, as to its path on leaving this organ. And perhaps the 

 inferior prolongation of the gastric cavity is what in other La- 

 mellibranchs has been called the duodenum. 



I saw no " crystalline style," neither did I detect the " tri- 

 cuspid body" ; the former most likely does not exist, the latter 

 has probably escaped me. 



EXPLANATION OF THE PLATES. 



PLATE in. 



Fig. 1 . View of right side of Chamostrea albida enclosed in the mantle : — 



a, anterior borders of the two lobes of the mantle turned back ; 



b, the point at which the borders are united ; c, pedal orifice ; 

 d, anterior adductor muscle ; e, borders of the ventral mai-gin 

 separated a little to show siphonal tubes ; /, exhalant tube ; g, in- 

 balaut tube ; h, posterior adductor ; i, posterior margin ; _;', extre- 

 mity of ossicle; Ic, membranous process surrounding same; I, notch 

 for the passage of tooth ; m, fourth minute aperture leading into 

 branchial chamber. 



Fig. 2. View of left side of animal, the mantle laid open : — a, body ; b, an- 

 terior adductor; c, pedal aperture; c?, inhalant siphon; e, exhalant 

 siphon ; /, fourth aperture leading into branchial chamber; g, gill- 

 plate ; g', rudimentary gill ; h, line dividing the two portions of 

 branchial organ ; i, foot ; j, mouth ; k, oral palps ; I, orifices of 

 interbranchial tubes ; m, membrane suspending gill ; n, the two 

 great posterior channels; o, two membranous Saps in connexion 

 with exhalant siphon ; p, the two extremities of ossicle ; q, pro- 

 cesses for secreting ossicle; r, notch in mantle for passage of 

 tooth. 



PLATE IV. 



Fig. 1 . Front view of mouth : — a, body ; j, oral orifice ; k, k, superior pair 

 of palps ; k', inferior ditto ditto ; I, anterior border of gill-plate, 

 exhibiting groove. 



Tig. 2. Siphonal tubes as they would appear when in action. 



Fig. 3. A portion of external surface of gill-lamina : — a, transverse plicse ; 

 b, extremities of plicae extending beyond lamina. 



Fig. 4. A portion of under surface of gill-lamina : — a, large transverse ves- 

 sels ; b, small vessels ; c, series of apertures. 



Fig. 5. Transverse section of lamina of gill-plate : — a, a, plicae on external 

 surface ; b, b, large transverse vessels seen on under surface of 

 lamina; c, c, small vessels of ditto ; d, d, series of apertures. 



Fig. 6. A portion of the wall of plicae highly magnified : — a, vessels run- 

 ning in the direction of plicae ; b, vessels running in the opposite 

 direction; c, apertures. 



Fig. 7. Margin of gill-plate much enlarged, exhibiting groove : — a, groove ; 

 b, b, extremities of pUcae of the opposite sides. 



