306 



DR. H. LYSTER JAMESON ON 



the respective peviotls, and with the relative rapidity of peri- 

 pheral growth in young and older oysters. Text-fig. 37 shows the 

 hypostracum of the same example (i)/. maxima) enlarged forty 

 ti mes. 



Text-fig. 37. 



njOLc. 



nxLcf 



The inner limit of the addnctor scar, in the same shell as that shown in text-fig. 36. 

 /;?/., hypostracum ; nac, nacre external to same ; nac.', nacre internal to 

 same. X 40. . 



Exactly the same relations occur in 31. vulgaris, a section 

 through the umbonal side of the adductor scar of which is shown 

 in PI. XXXIY. fig. 7. But in this example, an old thick Lingali 

 shell from the Persian Gulf, in which, in all probability, peri- 

 pheral growth, and consequently the wandering of the nuiscle, 

 had ceased, the hypostracum is thicker, measuring 130 /x in 

 thickness. 



The hypostracum undergoes but little secondary thickening, 

 compared with the nacre. Hence, in thick massive shells like 

 M. maxima, where the newly forming nacre in the umbonal 

 region and also towards the lip and around the muscle-scar out- 

 strips the hypostracum in development, the muscle-scar is the 

 thinnest part of the entire shell, except the extrerae edge. This 

 is well seen in text-fig. 36, in which figure the area between A 

 and B represents the muscle- impression, but it is even more 

 obvious in older thicker shells. Some interstratification of 

 hypostracum and nacre occurs at the borders of the muscle-scars, 

 where changes in the outline of the muscle have taken place. 

 This is figured by Tullberg in Mytilus (47, Taf. v. fig. 2). The same 

 is shown for Margaritifera vulgaris at Jiy.' , hy." , in PI. XXXIY. 

 fig. 7. Felix Miiller (29, Taf. xxix. fig. 13 h) shows the lateral 

 transition of this substance into nacre. I have observ^ed the 

 same thing in some of the " Muscle-Pearls " described below. 



When decalcified the hypostracum leaves behind it an organic 

 basis, which is somewhat different from the conchyolin of the 

 rest of the shell in its reaction towards stains. This is of interest 

 in connection with the view generally held that this laj^er, 

 unlike the other calcareous parts of the shell, which are probably 



