68 - COMPARATIVE ANATOMY CHAP. 



compared with those in the Ammonoidea, in which they are folded in a complicated 

 manner. 



Nautiloidea. In the following table we have the chief forms of the shell among 

 the Nautiloidea : ] 



(a) Orthoceras group. Shell straight or slightly bent. Silurian Trias. 



(b) Cyrtoceras group. Shell curved like a horn, but not regularly spirally 



coiled. Cambrian Permian. 



(c) Gyroceras group. Shell regularly spirally coiled, the coils, however, not 



touching each other. Silurian Permian. 



(d) Nautilus group. Shell regularly spirally coiled, the coils touching, or the 



outer clasping the inner. Silurian recent. 



(e) Lituites. Shell at first regularly spirally coiled, straightening later. 



Silurian. 



The siphuncle runs either through the centre of the septa, or through their 

 anterior or posterior sides. 



Ammonoidea. The shells of the (fossil) Ammonoidea are distinguished by very 

 complicated sutures, their zigzag lines are like the outlines of sharply-indented leaves 

 or richly -branched mosses, they are due to the extraordinary folding of the edges of 

 the septa, which are attached to the inner surface of the shell. The siphuncle is 

 always very thin in the Ammonoidea, and almost always pierces the septa on the 

 posterior side. 



The following quotation summarises the chief peculiarities in the form of the 

 Ammonite shell : 2 



"The shell, as a rule, forms a closed symmetrical spiral, the coils touching or 

 clasping one another. Some of the oldest forms are straight, or in youth incom- 

 pletely coiled. In certain groups of the Ammonoidea we find a tendency repeated at 

 different times (Trias, Jurassic, Chalk) to depart from the close symmetrical spiral, 

 and to adopt what are called accessory forms. The first step in this process of change 

 is in most cases the detachment of the occupied chamber from the next inner whorl ; 

 then, little by little, the inner whorls also separate, though they still remain in the 

 same plane the Crioceras stage. Sometimes the shell grows straight for a time, 

 then becomes hooked the Ancyloceras and Hamites stages, and, if only the occupied 

 chamber separates from the coiled part the Scaphites stage. Finally, entirely straight 

 shells arise in the Baculites stage. Rarely, the coils leave the symmetrical plane and 

 assume the shape of a snail's shell ; in this case, the shells may be either closely or 

 loosely coiled, the Turrilites stage." 



Dibranchia. The shells of all known Dibranchia, extinct or recent, are more or 

 less rudimentary, since they are never capable of sheltering more than a small portion 

 of the animal. Further, they are always internal, on the anterior side of the visceral 

 dome, and are overgrown by a fold of the integument. In Spirula (Fig. 33, p. 23) 

 alone, the shell is not completely overgrown, a portion at the apex of the visceral 

 dome remaining uncovered. 



The shell of the (fossil) Belemnites (Fig. 67 C) is straight, conical, and chambered; 

 the septa are near one another, and are pierced on the posterior or ventral side by 

 the thread-like siphuncle, which is enclosed in short, calcareous sheaths. The apex 

 of the shell (phragmocone) is protected by a conical calcareous sheath (rostrum or 

 guard), the only part usually preserved. The anterior wall of the last chamber 

 is produced downwards into a broad thin process, the pro-ostracum. 



In Spirulirostra (Fig. 67 D), the phragmocone begins to bend posteriorly (endo- 

 gastrically). The rostrum is triangular and pointed at the top. 



1 Steinmann-Doderlein, JSlemente der Paldontologie, 1890. 2 Ibid. 



