304 DR. H. LYSTER JAMESON ON 
Molluscan shell results, the axes of its constituent elements being 
approximately parallel. 
(iu1.) The Nacre. 
This substance, which forms the bulk of the shell, and gives 
the shells of the genus Margaritifera their commercial value as 
Mother-of-Pearl, and the pearls their beauty, is stratified, and 
in it the calcium carbonate is divided into extremely minute 
bodies in the organic network. It is secreted by the outer surface 
of the mantle and body-wall. 
I will not attempt here to review the many writings on the 
structure of this layer. J can at present add little to the recent 
work of Romer (32), who has studied its structure and that of its 
decalcified conchyolin framework very thoroughly. The organic 
basis which gives it its form, and which retains its iridescence 
after the calcareous salts have been extracted, consists of a series 
of parallel lamelle, of extreme fineness, united to one another 
at intervals by radial connections, so as to form a series of minute 
flat or lenticular chambers, separated by organic walls of extreme 
delicacy. The calcium carbonate appears to be enclosed in 
these chambers in the form of little polygonal plates or lozenges. 
This structure is difticult to observe, owing to the distorting 
effect of the decalcification process, which, owing to the evolution 
of gas-bubbles, tears some lamelle apart and forces others 
tightly together. It becomes much more obvious in some of 
the abnormal and pathological varieties of nacre described as 
“repair-substance ” below, notably in ‘ granular repair-nacre.” 
I believe that the lustre (not the iridescence) of mother-of-pearl, 
and of pearls, is in great measure due to the fact that each of 
these tiny plates is a minute, biconvex lens; and that the 
extraordinary and indescribable character of the light reflected 
from the surtace of a fine pearl is in part the cumulative expression 
of the action of these myriads of little lenses upon the light 
reflected from the surfaces of calcium carbonate and of conchyolin 
which underlie them. 
(iv.) Hypostracum, or Muscle-Attachment Substance. 
This curious substance has not secured all the attention it 
deserves. It has been described by several writers under the 
names Hypostracum (Thiele, 46), Stiibchenschicht (F. Miiller, 
29), durchsichtige Substanz (Ehrenbaum, 9; Tullberg, 47). 
T retain the name hypostracum, as emphasizing the distinct 
origin and characters of this layer, and as shorter and more 
convenient than ‘‘durchsichtige Substanz.” 
This hypostracum is a fine columnar layer forming the surfaces 
where the muscles are inserted into the shell (Pl. XXXIV. figs. 5 
& 7, hy.; text-figs. 36 & 37). 
It is more transparent than the nacre—indeed, the iridescence 
and lustre of the muscle-sear is due to the nacre lying below and 
[46] 
