152 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



G^lery modifications assisting in one way or another this actively 

 Table-ease moving life. The main result in all, however, is the enclosure 

 1. of the shell-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 fin -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 en sheathed by the mantle constitute a third Order, 

 corresponding, so far as living genera are concerned, to the 

 Dibranchia of Owen. For this Order the name COLEOIDEA 

 (sheath-forms) has been proposed ; the 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 points 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). 



The Ammonoidea also do not diverge greatly from the 

 general outlines sketched above. They do, however, break 

 up into a number of lines of descent which it has proved 

 exceedingly difficult 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 of 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). The further developments 

 of these foldings must be studied in the models and 

 specimens, and in professed text-books of palseontology. 



