500 EXTINCT GENERA OF TETRABRANCHIATA. 



genera. The Tetrabranchiata is the older group, comprising the 

 animals which dwell in chambered shells, and have a siphuncle 

 running through the chambers. This group is represented at the 

 present day by the Nautilus. The genera are chiefly distinguished 

 from each other by the mode of the growth of the shell, the character 

 of the margin of the septa, and the position of the siphuncle. These 

 differences are all connected with the development of the reproductive 

 organs, and positions of the muscle which holds the animal in the 

 shell. Two principal types are known, represented by the Ammonite, 

 which has the septa folded ; and the Nautilus, which has the septa 

 simple. The Nautilus dates from the Cambrian strata, and is met 

 with in all subsequent deposits. It becomes most specialised in 

 the form of the shell in the Carboniferous limestone, and in folding of 

 septa in the Palaeozoic genus Clymenia y and the Tertiary group Aturia, 

 found in the London clay : Discites ranges from Cambrian to Car- 

 boniferous ; Cryptoceras is Devonian and Carboniferous ; Temno- 

 cheilus and Trematodiscus are both Carboniferous ; Hercoglossa is 

 Cretaceous and Lower Tertiary ; Cinonia is confined to the Lower 

 Tertiary. Lituites and Trochoceras are both lower Palaeozoic genera ; 

 Gomphoceras, Phragmoceras, and Cyrtoceras are genera ranging from 

 the Cambrian to the Carboniferous ; Orthoceras ranges from Cambrian 

 to Trias, and includes multitudes of species. Gyroceras and Gonia- 

 tites range from the true Silurian to the Trias ; Rliabdoceras is Trias- 



Fig. 132. Nautilus. Fig. 133. Goniatites. Fig. 134. Ammonites. 



sic ; and though Ceratites is chiefly known from the Trias, it ranges 

 from the Devonian to the Chalk. 



The genus AMMONITES has been divided into sub-genera, distin- 

 guished by folding of the septa and form of the shell. Among them 

 are Sageceras, which is Permian and Triassic ; Arcestes, Didymites, 

 Lobites, Pinacoceras, Ptychites, Trachyceras, and Tropites, which are 

 exclusively Triassic ; JEgoceras is Triassic and Liassic ; A maltheus, 

 Lytoceras, Pliylloceras are Triassic, Jurassic, and Cretaceous ; Arietites, 

 Harpoceras, ^Ekotraustes, Oppelia, Peltoceras, Stephanoceras, Simo- 

 ceras, &c., are Jurassic; Avpidoceras, Cosmoceras, Haploceras, and 

 Perisphinctes are Jurassic and Cretaceous ; and Acanlhoceras, Olcoxte- 

 phanus, Schlceubachia, Stoloczkaia, &c., are Cretaceous. 



The genera which differ from Ammonites in their uncoiled mode of 



