64 



The retention of the deserted chambers and the interception of 

 certain spaces of the shell by calcareous septa, though not unknown 

 in the gastropodous univalves, is more common in bivalves. 



An oyster kept without food will frequently expend its last ener- 

 gies in secreting a new nacreous layer, at a distance fi-om the old 

 internal surface of the concave valve, corresponding to the diminu- 

 tion of bulk which it has experienced during its fast, and thus adapt 

 its inflexible out-ward case to its shrunken body. 



In the calcareous tube exuded from the elongated mantle of the 

 Septarice, Lam., the closed extremity of the tube is divided into 

 chambers by a succession of layers at a distance of half an inch 

 from each other, having a regular concavity towards the open ex- 

 tremity of the shell. These concave septa are composed entirely of 

 the nacreous constituent of the shell ; in one example which I have 

 examined, they were six in number ; they are thin, smooth, and 

 closely resemble the partitions in the Nautilus and Spirula save in 

 the absence of the siphonic perforation. 



Among Bivalves the Ostrece not unfrequently present shallow and 

 irregular chambers in the substance of the shell : the Etherice again 

 have vesicular cavities interposed between the testaceous laminse ; but 

 the most constant and remarkable example of the camerated struc- 

 ture of the shell is presented by a large Spondylusox Water-clam, so 

 called from the fluid which (until lost by slow evaporation) occupies 

 the chambers, and which is visible in the last-formed chamber 

 through the thin semitransparent exposed septum. 



In order to examine this camerated structure, and more esjjecially 

 to see how it was modified by the presence and progressive change 

 of place of the adductor muscle, I had a fine specimen sawn through 

 vertically and lengthwise. In the specimen now on the table, which 

 measures eight inches in length, the substance of the concave valve, 

 which is two inches one-third in thickness, at the thickest part in- 

 cludes fourteen chambers, separated from each other by very regu- 

 larly formed and stout partitions, composed, as in other chambered 

 shells, of the nacreous portion or constituent of the shell. The septa 

 are slightly undulating in their course, but present a general con- 

 cavity towards the outlet of the shell. Not any of these parti- 

 tions are, however, continued freely across the shell, but each be- 

 comes continuous at the muscular impression, which is near the 

 middle of the shell with the contiguous septa. In general, also, 

 the septa commence singly from the cardinal or upper wall of the 

 valve, and divide into two when about one-fourth of the way towards 

 the opposite or lower wall ; the thickness of the undivided part of 

 the septum being equal to, or greater than that of the two divisions 

 or layers into which it splits. 



We can readily understand why the septa must necessarily become 

 united together at the point of insertion of the adductor. The muscle 

 never quits its attachment to the valves ; •while the lobe of the mantle, 

 except in its circumference, and where it is attached to the adductor 

 muscle, must detach itself from the surface of the valve which is about 

 to be partitioned oiF, when it secretes upon the interposed fluid the new 



