306 DR. H. LYSTER JAMESON ON 
the respective periods, and with the relative rapidity of peri- 
pheral growth in young and older oysters. Text-fig. 87 shows the 
hy postracum of the same example (J/. maxima) enlarged torty 
times. 
Text-fig. 37. 


























STATA NTT 

The inner limit of the adductor sear, in the same shell as that shown in text-fig. 36. 
hy., hypostracum; nae., nacre external to same; nac.’, nacre internal to 
same. X 40. 
Exactly the same relations occur in J/. vulgaris, a section 
through the umbonal side of the adductor sear of which is shown 
in Pl. XXXIV. fig. 7. But in this example, an old thick Lingah 
shell from the Pen Gulf, in which, in all probability, peri- 
pheral growth, and consequently the wandering of the muscle, 
had ceased, the hypostracum is thicker, measuring 130 « 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-sear out- 
strips the hypostracum in development, the muscle-scar is the 
thinnest part of the entire shell, except the extreme edge. This 
is well seen in text-fig. 56, in Pnich 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 ficured by Tullberg in Mytilus (47, Taf. v. fig. 2). The same 
is Shown for Margar itifera vulgaris at hy.', hy.", in Pl. XXXIV: 
fic, 7. Felix Miiller (29, Taf. xxix. fig. 13) shows the lateral 
transition of this substance into nacre. I have observed 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. his is of interest 
in connection with the view generally held that this layer, 
unlike the other calcareous parts of the shell, which are probably 
(48) 
