152 GUIDK TO THK l-'OSSIL INVERTEBRATE ANIMALS. 
Gallery 
VII. 
Table-case 
1 . 
inodiliciitions a.ssisting in one way or another actively 
moving life. The main result in all, liowever, is the enclo.snre 
of the sliell-cone by the mantle-folds and its reduction in 
size, so that from being an external protection to the animal 
it becomes an internal support for the mantle and for Un-like 
appendages. This does not mean that it becomes an internal 
skeleton in the true sense of the word ; for its relations to 
the visceral hump always remain the same : it is always 
outside the true body-wall. Cephalopods in which the shell 
is thus eiTsheathefl by tlie mantle constitute a third Order, 
corresponding, so far as living genera are concerned, to tlie 
Dibranchia of Owen. For this Order the name COLEOIDEA 
(slieath-forms) has been proposed ; tlie name BELEMNOIDEA 
has also been used for it, but is more frequently restricted to 
one of its subdivisions. 
In their further history the Nautiloidea undergo no 
changes of importance. It will be well, however, to study 
the preparations showing some jioints in their structure. 
Among these are some internal casts and some portions of 
.shell, showing the scar made by the attachment of the 
muscles that fix the body to the wall of the shell (Fig. 82 a). 
'file Ammonoidea also do not diverge greatly from the 
general outlines sketched above. 'They do, however, lireak 
up into a number of lines of descent which it has jiroved 
exceedingly diihcult to unravel. In this study great import- 
ance is attached to the foldings of the suture, and some 
attention may profitably be given to the models showing 
how the chief types of suture are gradually developed from 
the simple type of the older forms. A fold of the suture 
directed towards the opening of the shell is called a saddle, 
and a fold in the opposite direction is called a lobe. The 
absence of folding in the earlier sutures characterises forms 
known as Asellati (unsaddled). Folding, when once started, 
begins at an early stage in the life-history ol each shell, and 
is manifest even in the suture between the protoconch and 
the first chamber (Fig. 81 f-h)- Forms in which this suture 
has one broad saddle, and in which external lobes and 
saddles appear gradually in the later sutures, are called 
Latisellati (broad-saddled). Those in which this suture has 
a narrow saddle in the middle line, bounded on each side by 
a lateral lobe, and this again by a lateral saddle, are called 
Angustisellati (narrow-saddled). 'I’lic further developments 
of these foldings must be studied in the models and 
specimens, and in professed text-books of paheontology. 
