182 



THE LTAS AMMONITES. 



marginal septal foliations are well shown, we observe that 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 

 Amaltheus margaritatus, Montf. 



Fig. 42. — Mould of the lobes in 

 Amaltheus pustulatus, Miinst. 



Fig. 43. — Disposition of the lobes in 

 Phylloceras heterophyllum. Sow. 



sutures; when they are folded, the elevations are called saddles, and the inter- 

 vening depressions lobes. In the Ceratites (figs. 37 — 40, and 44) these parts are very 



Fig. 44. — Disposition of the lobes in 

 Ceratites nodosus, De Haan. 



Fig. 45. — Form of the subdivisions of 

 the lobes in Ammonitidse. 



FiG. 46. — Mould of the disposition 

 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 highly complicated accordingly. 



In the dwelling-chamber of AmmonitidcB we sometimes find a remarkable body, 

 having some resemblance to a bivalve shell widely opened ; this is called the Aptychus, 

 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 Aptychus of 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 figured, and will be 



