NATURAL HISTORY. 
121 
GALLERY.] 
lar mouth. Here, I believe, should be placed for the 
present, on account of the similarity of the form and 
texture of the shell, the Paper Nautilus ( Argonauta ) 
and probably the fossil genus Bellerophon, As yet only a 
peculiar kind of cuttle fish, with a web to the end of the 
longer arms, has been found in the Argonaut shells; but there 
are many reasons for believing that this is only a parasite, 
adapted by its form to live in such shells, as the web 
of the arms is used by the animal to embrace the shell 
and keep it in its right position on the body; for, unlike 
all other mollusca which form the shell they inhabit, the 
cuttle-fish is not attached to the shell by any muscle, nor 
has the animal any muscle like the bone-bearing cuttle¬ 
fish, formed for the purpose of attaching the body to its 
internal shell. Secondly, the animal when alive does 
not fit the shell, so that the shell cannot have been moulded 
upon its body, as in other mollusca. Thirdly, the skin of 
the animal is of the same texture and appearance as the 
other naked cuttle-fish, and the presence of sand between 
the shell and the body appears to cause no uneasiness to 
the animal, as it does in all other shell-bearing mollusca, 
where the animal immediately rids itself of the irritation so 
caused, by covering it over with a calcareous coat. Lastly, 
the animals found in these shells are always females, 
and the apex of the shell is filled with very small eggs, 
while from the large size of the young shell which is to be 
seen on the apex of the true Argonaut, we should expect 
the animal which formed that shell, to have a large egg. 
The other animals of this order have their branchiae 
placed on the right side of the body, in the groove between 
the edge of the mantle and the foot. They comprehend 
the two following families. 
The family of Pleurobranchidce, consisting of the ge¬ 
nera Berthella and Pleurobrcinchus, have a very thin 
membranaceous shell inclosed in the mantle. 
The family of the Umbrellidce have a suborbicular, flat, 
hard, external shell. The genus Umbrella was formerly 
supposed to have its shell placed on its foot, and was called 
Gastroplax , but this has been proved to be an error. Tylo - 
dina differs in the head being produced and bifid; while 
in Umbrella it is sunk into a deep cavity in the front of 
the foot. 
Q 
