248 Lamarck's Genera of Shells. 



in the different genera. Some are discoidal, with spiral whorls 

 either visible or enveloping ; some form a turrited, pyramidal spire ; 

 whilst others are straight, or nearly so, without any spire at all. 

 1. Ammonites*. 



Shell discoidal, spiral, whorls contiguous, and all of them visi- 

 ble ; the interior parietes articulated by sinuous sutures. Septa 

 transverse, lobed and indented at the circumference ; their discs 

 without any siphon, but perforated by a sort of marginal tube. 



The ammonites differ essentially from the nautili, by the sinuous 

 sutures of the internal parietes, and by the similarly sinuous form of 

 the septa. From the orbulites, by all the whorls being distinctly 

 visible. 



The ammonites are only known in the fossil state, and most of 

 the specimens, found in our collections, are merely internal pyritic 

 casts of the shells. They are common in almost all countries 

 chiefly in schistose or argillaceous formations, and M. Menard 

 found one in the maritime Alps, at an elevation exceeding 9000 

 feet. Several species are of very large size. They abound so 

 much in Burgundy, that the road between Auxerre and Avalon is 

 mended with ammonites. 



Type. Ammonites Kunigif. 



Shell discoid, convex, with radiating undulations ; inner whorls 

 half exposed; marginal undulations numerous; central undulations 

 few, very prominent ; aperture cordate elongated. PI. vi. Fig. 231. 

 From KellowaysJ. 



2. Orbulites §. 



Shell subdiscoidal, spiral, whorls contiguous, the last envelop- 



* From amnion, a name of Jupiter, who was worshipped in Libya under the 

 form of a ram. The old name of the ammonites was cornu ammonis, from 

 their resemblance to a ram's horn. 



-f- Named in honour of Mr. Kdnig. 



X Our figure, and the preceding specific character, is taken from the Mineral 

 Conchotomy of the late James Sowerby, Esq., in whose lamented death natural 

 history has recently experienced a severe loss. The talents and ardour of his 

 eons, happily forbid our deploring it as irreparable. 



Laniark describes 20 species of ammonites, hut he has not given a single refe- 

 ence to any other author, or to any figure, for either of the species described. 



§ From orbis, an orb. 



