MORPHOLOGICAL DIFFERENTIATION. 335 



tilus), circular (Cyrtoceras), elliptical (Trochoceras, Phragmocus), or some ■ 

 times cruciform. 



The initial chamber of Clymenia, the Goniatites, and the Ammonites 

 is formed in an entirely different manner. In all these this spirally en- 

 rolled chamber has a vesiculous, spherical, or ovoid form, generally a 

 little depressed and transversely striated; no scar or impression has been 

 met with, and the siphon begins at the anterior wall. It is not probable 

 that the initial chambers of the form of a truncated cone of the Nautilidze 

 are homologous with the spherical enrolled initial chambers of the Am- 

 monitidae; on the contrary, the presence of a cicatrix makes it probable 

 that this impression represents either the point of attachment, or the 

 opening of communication, closed at a later stage, of a frail vesicle, per- 

 haps membranous, which corresponds with the initial chamber of the 

 Ammonites. According to this view, proposed by Hyatt, the initial cham- 

 ber of the Nautilidae should be equivalent to the second chamber of the 

 Goniatites and the Ammonites. 



The Siphon is a tubular prolongation of the skin of the posterior pan 

 of the body; it traverses all the chambers, and in Nautilus begins under 

 the form of a closed tube covered with nacre, in the initial chamber, or 

 truncated cone, where it touches the internal posterior wall at the same 

 place, where exteriorly is seen the cicatrix. In the Ammonites and the 

 Goniatites the siphon begins with a spherical swelling situated imme- 

 diately behind the anterior wall of the initial vesicle (nucleus), conse- 

 quently perforating only the first septum, without penetrating more 

 deeply into the chamber. According to Hyatt, the part of the siphon 

 penetrating into the embryonal chamber was, in general, only a depression 

 of the first partition. Munier-Chalmas has observed in the Ammonites 

 a particular prolongation of the siphon in the initial chamber which ought 

 to have replaced the true siphon in the 

 embryonic stage; this prosiphon, as he 

 calls it, is attached to the siphon, which 

 begins in a reflected cul-de-sac, and is 

 very variable in form. It forms some- 

 times an enlarged membrane, sometimes 

 a cylindrical tube; the prosiphon does 

 not communicate with the siphon, prop- 

 erly speaking. 



In the recent Nautilus the siphon is 

 a rather solid membranous tube covered 

 exteriorly by a thin coating of brown 

 color, earthy, formed of fine calcareous 



grains. In the Ammonites (see Fig. 102) ^"^- '99 -^ledian section of a youns: Am- 



. \ b I monite ( 7Vci^//<rj) snowing the siphonal 



this exterior calcareous envelope seems collar and its change of direction on pass- 



tr^ tol-o r^n n ,v,^-„ „,,i,„. »' 1 ■ .. '"K frooi the initial chamber <!, where it 



to take on a more substantial consist- is retrosiphonate or turned backwards, to 



ency, so that the siphon is enclosed in a ^^^ third whorl, where it is prosiphonate 



J 1- . , , - . or turned forwards. (After Zittel.) 

 delicate calcareous tube. It is necessary 



not to confuse this envelope of the siphon itself with that which is called 

 the siphonal collar, which is met with always where the siphon penetrates 

 the septum. 



The siphonal collar (Fig. 109) is a reflection or production of the sep- 

 tum of greater or less length, directed, generally, in Nautilus, backward, 



