of Spondylus varius. 



409 



chambers, may not be unacceptable to your conchological 

 readers. 



In order to examine in what manner the camerated struc- 

 ture of this shell was modified by the presence and progres- 

 sive change of place of the adductor muscle, I had the fine 

 specimen here represented of half the natural size, {jig. 21), 



Section of Spondylus varius. 



sawn through vertically and lengthwise. The length of the 

 concave valve is eight inches; it is two inches one-third 

 in thickness, and includes fourteen chambers, separated from 

 each other by very regularly formed and stout partitions, com- 

 posed, as in other chambered shells, of the nacreous portion 

 or constituent of the shell. The septa are slightly undulat- 

 ing in their course, but present a general concavity towards 

 the outlet of the shell. Not any of these partitions are, how- 

 ever, continued freely across the shell, but each becomes con- 

 tinuous 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 un- 

 divided 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 necessa- 

 rily 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 circumfe- 

 rence, and where it is attached to the adductor muscle, must 

 detach itself from the surface of the plate which is about to 

 be partitioned off, when it secretes upon the interposed fluid 

 the new partition or basis of support. It is obvious, therc- 



pp2 



