REPTILIAN AGE. JURASSIC PERIOD. 121 



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

 to be impressed with its resemblance to a jaw or beak. Indeed, so striking is this 

 analogy, that we are strongly inclined to adopt that conclusion, notwithstanding the 

 fact that we must then view the two^ enveloping valves as forming together one of 

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

 opercula — 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 aU the existing Cephalopoda are known to be provided 

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

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

 is received within, and enveloped by, the lower, much as the appendage we have 

 described lies between the valves in our specimen. 



Genus AMMONITES, Bruguiere. 



Synon. — Ammonites, Brugiiiere, Encyc. Mefh. I, 1789, xvi and 28. — Lamarck, Prodr. 1799, 80 ; Syst. Ann. 1801, 

 100; Phil. Zool. 1809, 323.— Feross. Tab. Syst. 1819.— Roisst, Mol. V, 1805, 16, &c. 



Planulites, Montfort, Conch. Syst. I, 1808, 78; (not Lamk. 1801 ? ; nor Monster, 1832.) 



ElUpsolithes, Montf. ib. 86. 



Argonauta, Reinecke, Mar. proto. Naut. 1818, * * * (not Lin.). 



Ammonita, Gray, Lond. Med. Rep. 1821. — Fleming, Brit. Ann. 1828, 240. 

 Etym. — Amman, a name of Jupiter. 

 Exam]}, — Ammonites hisulcatus, Brdguiere. 



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

 ous or embracing at all stages of growth, and coUed 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 Goniatites. They 

 differ 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 are, 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 ScapJiites, from which they only 

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



* It is worthy of note in this connection, that M. Coquand has maintained that an Aptychus fos 

 hitherto understood) properly consists of a single piece — that the apparent existence of two dist^ct 

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

 from accidental pressure. 

 16 January, 1865. 



