376 CAUSES OF PARTIAL PRESERVATION. 
some traces of it must have remained in those 
beds of lias at Lyme Regis, which are loaded 
with Nautili and Ammonites, and have preserved 
the ink of naked Cephalopods in so perfect a 
condition. The young Sepia officinalis, whilst 
included within the transparent egg, exhibits 
its ink-bag distended with ink, provided before- 
hand for use as soon as it is excluded ; and this 
ink-bag is surrounded by a covering of brilliant 
nacreous matter, similar to that w^e find on cer- 
tain internal membranes of manv fishes.* 
* I would here add a few words in explanation of the curious 
fact, that among the innumerable specimens of Belemnites which 
have so long attracted the attention of naturalists, not one has 
till now been found entire in all its parts, having the ink within 
its external chamber ; either the fibro-calcareous sheath is found 
detached from the horny sheath and ink-bag, or the ink-bag is 
found apart from the Belemnite, and surrounded only by the 
nacreous horny membrane of its anterior chamber. We know 
from the condition of the compressed nacreous Ammonites in the 
Lias-shale at Watchet, that the nacreous lining only of these 
shells is here preserved, whilst the shell itself has perished. This 
fact seems to explain the absence of the calcareous sheath and 
shell in almost every specimen of ink-bags at Lyme Regis, which 
is surrounded with iridescent nacre, like that of the Ammonites 
of Watchet. The matrix in these cases may have had a capacity 
for preserving nacreous or horny substances, whilst it allowed the 
more soluble calcareous matter of shells to be removed, probably 
dissolved in some acid. 
The greater difficulty is to explain the reason, why amidst the 
millions of Belemnites that are dispersed indiscriminately through 
almost all strata of the Secondary series, and sometimes form 
entire pavements in beds of shale connected with the Lias and 
Inferior oolite, it so rarely happens that either the horny sheath, 
or the ink-bag, have been preserved. We may, I think, explain 
