THE CEYLON PEARL-OYSTER. 305 



sliining through this " tlurchsichtige Substanz," the sul)stance 

 itself not possessing the structure to which these optical properties 

 are due. 



It is composed of columnar or fibrocrystalline 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 

 whicli remains on decalcification (P). 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 occurs 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 adAancing 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-fio-. 36. 



Margai'ififera mct.riwa Jameson. A young shell or "chicken shell" from Port 

 J3ar\vin, Northern Territory of Australia (LonJon markets). Section from 

 the uml)o to the shell-marfjin passing- through the middle of the adductor 

 impression, pr., prismatic layer ; A-B, muscle-scar, covered with " Hypo- 

 stracum " ; Jij/., the hypostracum layer, by means of which the migration of 

 the muscle, from what is now the umbonal region, witli the growth of the 

 shell can be traced ; nac, nacre of the shell-margin, formed external to the 

 nuiscle-scar; nac.', nacre of the thick subumbonal region, deposited internally 

 to the hypostracum. Two-thirds of natural size. 



In the shell figured, a young example of the large white 

 Australian Mother-of-Pearl shell (31. maxima Jameson), the 

 hypostracum is 18-20 ^ thick over the muscle-scar, thinning 

 out to 10 ^ and then to 4 or 5 /a 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. Zool. Soc— 1912, No. XX. 20 



