458 THE ANIMAL KINGDOM. 



horny or calcified, as ' pro-ostracum.' The chambered shell is not de- 

 veloped in any living Decapoda except Spirula. The part homologous 

 with the guard is alone present. It constitutes the sepiostaire, or cuttle- 

 bone of Sepia, composed of calcareous lamellae inclosing air spaces, and 

 the horny pen, or gladius of Loligo, composed of conchiolin. 



The shell of Tetrabranchiata, the living Nautilus and the extinct 

 Nautiloidea and Ammonoidea, is external and is secreted by the mantle : 

 but nothing is known as to the initial phase of its development. It is 

 either straight, loosely or closely coiled, and then either in a spiral like 

 a snail, or more generally in the same plane as in Nautilus and many 

 extinct forms. It is chambered, and the last or largest chamber is the 

 one occupied by the animal which is attached to it by a muscle. Succes- 

 sive chambers are separated from one another by septa, and the unoccupied 

 chambers are filled with air. The external edges of the septa are simple 

 in Nautiloidea, but in the older chambers of the Ammonoidea are thrown 

 into folds. A forwardly projecting fold is known as a ' saddle : ' one that 

 projects backwards as a ' lobe.' The edges of the folds are generally not 

 simple but wavy, sometimes in a highly complex manner. The chambers 

 are traversed by a tube the sipho or siphuncle, which has membranous 

 walls, prolonged from the integument, protected however by an outer 

 coating of nacre continuous with the substance of the septa. This 

 nacreous coat or ' collar ' may form a complete investment or only a 

 partial one to the membranous siphuncle. In the latter case it projects 

 backwards from the septa in nearly all Nautiloidea, forwards in Ammonoidea, 

 except sometimes Jn the first chambers. The siphuncle is placed centrally 

 in most Nautiloidea, but it may be either near the concave or the convex 

 side of the shell. The latter is its position in Ammonoidea, except 

 occasionally in the first chambers. In Nautilus the convex side of the 

 shell corresponds to the ventral aspect of the animal, as it appears to do 

 in Ammonoidea ; but extinct Nautiloidea vary in this respect, even within 

 the limits of the same genus. 



The first chamber of the shell or protoconch has certain special 

 features. In Ammonoidea, as also in the chambered shell of Dibranchiata, 

 it is enlarged as compared with the next chamber ; its anterior wall is 

 bulged inwards by the sipho which does not extend through it ; it is 

 slightly dilated and covered with nacre forming the nucleus ; and is often 

 traversed by a 'pro-sipho' not connected with the siphuncle. In the 

 Nautiloidea it is small as compared with the next chamber and only 

 slightly curved, not coiled as in the coiled Ammonoidea, and the siphuncle 

 traverses it and commences at its apex, which is marked externally by 

 a 'cicatrix.' The presence of this cicatrix has been explained on the 

 supposition that there is a deciduous protoconch (Hyatt). The differences 

 observable have led some authorities to associate the Ammonoidea with 



