]82 
THE LTAS AMMONITES. 
marginal septal foliations are well shown, we observe tliat all their points are 
directed away from the aperture (figs. 41 and 43), and the concavities or rounded 
parts are directed towards the aperture (fig. 45). The outlines of the septa are termed 
Fig 41.— Disposition of the lobes in Fig. 42. — Mould of the lolies in Fig. 43. — Disposition of the lobes in 
Amaltheus margaritatus, Montf. Amaltheus pustulatus, Miinst. Phylloceras heterophyllum. Sow. 
sutures; when they are folded, the elevations are called saddles, and the inter- 
vening depressions lohes. In the Ceratites (figs. 37 — 40, and 44) these parts are very 
Fig. 44.— Disposition of the lobes in Fig. 45. — Form of the subdivisions of Fig. 46. — Mould of the disposition 
Ceratites nodosus, De Haan. the lobes in Amnaonitidae. of the lobes of Phylloceras. 
simple, and here we see the rounded saddles directed forwards and the dentated lobes 
turned backwards ; the same is the case in Amaltheus margaritatus (fig. 41), and 
Phylloceras heterophyllum (fig. 43), with this difference, that in all the Ammonitidse the 
sutures are very long, and the lobes and saddles Jiighly complicated accordingly. 
In the dwelling-chamber of Ammonitidm we sometimes find a remarkable body, 
having some resemblance to a bivalve shell widely opened ; this is called the Apti/cluis, 
which has been the subject of much speculation and difference of opinion as to what it is and 
what it did in the economy of the Ammonite. Its form and structure differ materially 
in the different species, so that whenever the Aptychusoi an Ammonite is found in position 
it ought to be correctly drawn and accurately described. In the plates of this work 
several authenticated forms in connection with their shells are fiarured, and will be 
