THE CEYLON PEARL-OYSTER. 301 
layer of the periostracum is closely adherent to the specialised 
epithelium, so that secondary thickening, if it takes place (and 
my own observations on Mytilus and Modiola lead me to think 
that it does so to some extent, a view which is held also by 
List, 27 6, p. 55), differs from that of all other parts of the shell 
(including the inner layers of the periostracum) in that it 
is secreted from outside or centrifugally with respect to the 
body of the animal and the shell, instead of from inside or 
centripetally. It would, indeed, seem as though there were 
morphological grounds for restricting the name periostracum to 
this particular layer of the outer cuticle-like substance, or, failing 
that, for introducing a term which would separate it more sharply 
from the more bulky inner layers. The structural distinctions of 
this layer are well shown by Romer (32) fig. 25 (Margaritana), by 
Moynier (28) fig. 50 (Mytilus), by Tullberg (47) Taf. iv. fig. 3, 
fig. 4d (Mytilus), and by List (27 6) in the Mytilide generally. 
The greater part of the periostracum, however, is Jaid down 
centripetally, layer upon layer, by the epithelium on the outer side 
of the marginal groove (i.e. on the inner side of the outer of 
the two lobes of the mantle-margin, Tullberg (47) p. 27). It is 
stratified, and in Mytilus contains, near its outer limit, a charac- 
teristic layer of largealveoli. The stratifications of this substance 
have been shown by Romer to correspond to layers of minute 
alveoli (32, fig. 25). 
Internally the inner layers of the periostracum pass over into 
the conchyolin framework of the prismatic layer. 
In some forms, e. g. Anodonta, the distinction between the two 
constituents of the periostracum are emphasized by the outer 
layer being much greater in area than the inner ones, and being 
thrown into folds upon which the inner layers lie unconformably. 
The Mother-of-Pearl Oysters (together with such forms as 
Ostrea and Pecten) differ from the types to which the above 
description refers in the fineness of the periostracum and in having 
much more freely retractile mantle-margins. In sections of the 
decalcified shells of the Mother-of-Pearl Oysters it is difticult— 
indeed, I might say impossible—to differentiate the periostracum 
from the outer layer of the organic basis of the prismatic sub- 
stance (text-fig. 35, p. 302; see also Pl. XXXIV. fig. 6). Here 
marginal growth proceeds by a series of steps and retreats, the free 
mantle-margin being retractile to the edge of the nacre, and being 
so retracted when the shell closes. Thus, after a new process 
of the lip has been formed, the mantle-margin is withdrawn, 
and forms a fresh attachment on the inner surface of the last- 
formed lip, from which a fresh lip is produced. So the periostracum 
of lip no. 2 is attached to and apparently a direct continuation of 
the inner surface of the conchyolin of the prismatic layer of lip 
no. 1, and constitutes the outer layer of the conchyolin of the 
prismatic layer of lip no. 2. But Herdman has shown that, 
difficult. as it is to distinguish a separate periostracum in sections 
of the shell, such a layer, of extreme delicacy, does exist at the 
[43] 
