PHYLUM MOLLUSCA 



795 



curvature), that of Nautilus forwards (exogastric curvature). More- 

 over the shell of Spirilla is an internal structure, being almost 

 completely covered by the mantle. 



The shell of the extinct Ammonites (Fig. 694), which are usually 

 referred to the Tetrabranchiata, resembles 

 that of the Nautilus in many respects, 

 being a chambered spiral shell with a 

 large terminal chamber, and with a 

 siphuncle. The chief external difference 

 is in the form of the sutures, or lines of 

 union of the edges of the septa with the 

 side wall of the shell ; these are more or 

 less complexly lobed, instead of being 

 entire as in Nautilus. But in one im- 

 portant respect the shell of an Ammonite 

 differs from that of Nautilus and ap- 

 proaches that of the dibranchiate Spirula. 

 At the apex of the spiral is an initial 

 chamber or protoconch, which is dilated 

 and separated from the first of the or- 

 dinary chambers by a constriction, and 

 has passing into it a prosiphon not con- 

 tinuous with the siphuncle. The Am- 

 monite was also characterised by the 

 possession of a paired or unpaired struc- 

 ture, sometimes horny, sometimes cal- 

 careous, called theaptychus,riot represented 

 in any existing form. The aptychus, 

 which was composed of two parts, may have been of the nature 

 of an operculum for closing the mouth of the shell, but was 

 more probably endoskeletal. Young Ammonites, each with its 



aptychus, have been found within the 

 shell of the parent, in which they 

 must have remained protected during 

 their development. 



In the ordinary decapod Dibranch- 

 iata the shell may consist of three 

 parts — a horny pen or pro-ostracum, a 

 calcareous guard, and a part termed 

 the phragmacone. The last, which 

 alone represents the shell of Spirula, 

 has the form of a cone divided intern- 

 ally by a series of septa perforated by 

 a siphuncle. These parts* are most 

 completely developed in the extinct genus Belemnites, in which 

 the shell (Fig. 695) consists of a straight, conical, chambered 

 phragmacone (phr.), with a siphuncle, enclosed in a calcareous 



1 ic . 60S.— Spirula peronii, 

 lateral view, d, terminal 

 sucker ; /. funnel ; », **, *-. 

 projecting portions of the 

 shell, the internal part of 

 which is indicated by dotted 

 lines. (Krom Cooke.) 



Fit;. 004.— An Ammonite (Ceratites 

 nodosus). 



