P, K I'll I.I A \ AGE. JURASSIC PERIOD. 



In examining the curious third appendage described nlxive, one can scarcely fail 

 t<> !>< impressed with it- resemblance to a jaw or beak. Indeed, so striking is this 

 analog, that we are strongly inclined to udopt that conclusion, notwithstanding the 

 fact that we nm>t then \ iew the tuo' en\eloping valves as forming together one of 

 the opposing inandililes. The opinion that these bodies may be jaws instead of 

 opercnia first suggested by Van Breda, if we mistake not receives additional 

 support from the entire absence, so far as known, of anything else representing jaws 

 or beaks, within the thousands of Ammonites that have been broken open in various 

 parts of the world; while all the existing Cephalopoda are known to be provided 

 with such oral organs. Again it will be remembered, that in the living Naitfiliix 

 (the beaks of which are partly calcareous, and partly corneous), the upper mandible 

 i- received within, and enveloped by, the lower, much a8 the appendage we have 

 described lies between the valves in our specimen. 



Genus AMMONITES, BRUGUIERE. 



Synon. Ammonite*, BBCOCIF.BB, Encyo. Meth. I, 1789, zvl and 28. LAMABCK, Prodr. 1799, 80; Syst. Ann. 1801, 

 100; Phil. Zool. IsO'.l, 323. FBBCM. Tab. Sygt. 1819. ROIBST, Mol. V, 1805, 16, Ac. 



Ptanulite*, Ifontfort, Conch. Sygt. I, 1808, 78 ; (not LAME. 1801 ? ; nor MCJCSTBB, 1832.) 



ElliptolHku, Montf. ib. 86. 



Aryonaula, KKIMO KB, Mar. proto. Naut. 1818, * * * (not I.ix.)- 



Ammouiin, ORAY, Lou.l. Med. Rep. 1821. FLEMIXO, Brit. Ann. 1828, 240. 

 Elym. Amman, a name of Jupiter. 

 Kxamp. Ammonite* bitulcatut, BucociEBB. 



Shell discoidal or more or less convex, sometimes subglobose. Volutions contigu- 

 ous or embracing at all stages of growth, and coiled in the same plane ; umbilicus 

 varying greatly in breadth and depth with the species. Surface costate, nodose, 

 subspinous, striate, or smooth. Lip simple, inflected, or with various lateral ap- 

 pendages. Lobes and saddles of the septa more or less branched and deeply 

 divided ; the margins of the subdivisions sinuous and dentate. 



In form, the dorsal position of the siphon, and often in ornamentation, the Am- 

 monites present scarcely any difference from the Ceratites and Goniatitcs. Tiny 

 ditt'er from the latter, however, in having the lobes and saddles of the septa divided 

 and variously branched or dentate, instead of simple. From the former they often 

 present but slight and scarcely perceptible differences, even in the septa, the lobes 

 of which only differ in being more or less deeply divided and branching, instead of 

 merely serrated on their margins. There arc, however, some intermediate species 

 connecting these groups, so that even palaeontologists do not always agree in regard 

 to their position. 



The Ammonites are also related to the genus &-<ijihit>, from which they only 

 differ in not having the last or body whorl of the adult shell deflected from the 



1 It is worthy of note in this connection, that M. Coqnand has maintained that an Aptychus (as 

 hitherto understood) properly consists of a single piece that the apparent existence of two distinct 

 valves, is produced by the fracture of a single flexed plate, along a mesial line of least resistance, 

 from accidental pressure. 



1U January, 1865. 



