THE CEYLON PEARL-OYSTER. 305 
shining through this ‘“ durchsichtige Substanz,” the substance 
itself not possessing the structure to which these optical properties 
are due. 
It is composed of columnar or fibroerystalline needles of 
carbonate of lime (Stiibchenschicht, Miiller), but shows in 
places, in addition to its columnar structure, a distinct strati- 
fication parallel to the surface; this is seen also in the basis 
which remains on decalcification (Pl. XXXIV. fig. 5). I attri- 
bute this stratification to variations in the organic basis, which 
are probably independent of the form and structure of the 
crystalline needles. Hypostracum only oceurs where the 
specialised muscle-attachment epithelium is inserted into the 
shell, and, as the muscles move away from the umbonal region 
with the growth of the shell, it is quickly covered over by ordinary 
nacre which is deposited in the wake of the advancing muscle. 
In a section of the shell from the umbo through the adductor 
scar the hypostracum layer can be traced across the shell through 
the nacre from the scar to the umbo, the thickness of the over- 
lying nacre increasing as the umbo is approached, By means of 
this hypostracum layer, the wandering of the adductor muscle is 
recorded in the shell-substance (text-fig. 36, hy.), 
Text-fig, 36. 



Margaritifera maxima Jameson. A young shell or “chicken shell”? from Port 
Darwin, Northern Territory of Australia (London markets). Section from 
the umbo to the shell-margin passing through the middle of the adductor 
impression, pr., prismatic layer; A—B, muscle-scar, covered with “ Hypo- 
stracam”; hy., the hypostracum layer, by means of which the migration of 
the muscle, from what is now the umbonal region, with the growth of the 
shell can be traced; nac., nacre of the shell-margin, formed external to the 
muscle-sear; nac.’, nacre of the thick subumbonal region, deposited internally 
to the hypostracum, ‘I'wo-thirds of natural size. 
In the shell figured, a young example of the large white 
Australian Mother-of-Pearl shell (Jf. maxima Jameson), the 
hypostracum is 18-20 « thick over the muscle-scar, thinning 
out to 10» and then to 4 or 5 w at the extreme outer edge of 
the scar, where the muscle has most recently made attachment. 
As this layer is traced backwards towards the umbo, through the 
nacre, it is found to get gradually thinner, just as the prismatic 
substance (which in this shell is about 1 mm. thick in the region 
of the adductor scar, and in the lip of very old examples of the 
same species may be 2 or 3 mm. in thickness) is found to get 
thinner towards the umbonal region, These differences are no 
doubt associated with the relative ages and sizes of the animal at 
Proc. Zoou. Soc.—1912, No. XX. 20 
[47] 
