CLASS V RANGE OF THE AMMONOIDEA 673 



considerably further back into tbe Carboniferous system. These Paleozoic Aiimionoids 

 stand in the development of their septa between the Goniatites and the more highly 

 specialised Mesozoic Ammonites. 



With the beginning of the Mesozoic era, the true Ammonites developed with 

 great rapidity. In the middlq European Muschelkalk only the genera Geratites, 

 Bcneckeia, Hungarites, Balafoniies, Arniotites, Acrochordiccras and Ptychites have as 

 yet been discovered. On the other hand, in the Alps, Spitzbergpn, the Himalayas, 

 in western North America and 'in Siberia, there have been found great numbers of 

 Ammonites in rich faunas of the Lower, Middle and Upper Triassic. The families 

 of Tropitidae, Ceratitidae, Ptychitidac, Cladiscitidae and Pinacoceratidae belong 

 exclusively to the Triassic ; the Arcestidae begin in the Coal Measures, but reach 

 their greatest development in the Triassic. 



In the development of their septa the Triassic Ammonites show an unexpected 

 variety of form and complexity. Certain genera {Sageceras, Lccanites, Lohites) scarcely 

 pass the goniatitic stage of development ; many others (Meekoceras, etc.) only reach the 

 ceratitic stage. In the Arcestidae, Tropitidae, Cladiscitidae, Ptychitidae and Phyllo- 

 ceratidae the lol^es and saddles have become digitate. Indeed, in Pinacoceras is found 

 the greatest complexity of development of the septa that has been observed among the 

 Ammonites. Along with the typical forms the Upper Triassic of the Alps has fur- 

 nished also a number of reversionary types or aberrant forms (Gochloceras, Rhahdoceras, 

 Ghoristoceras), which are distinguished by reduction of the septa to great simplicity. 



■ With the Lias a fundamental change in the Ammonites occurred. Of the 

 numerous Triassic genera and families, with the exception of the Phylloceratidae, all 

 have come to an end and are replaced by new forms. The causes that made the 

 Cephalopods so rare in tlie Rhaetic are unknown. It may be that not all these 

 groups were extinguished, but that they lived on in other, as yet unknown regions, 

 and when we next see them in the Jurassic they have changed beyond recognition. 



In the Lower Lias the Aegoceratidae are almost the only forms ; the -genera 

 Psiloccras, Arietites and Schlotheimia, are confined to this stage. In the Middle Lias, 

 along with the Aegoceratidae, are represented the Harpoceratidae, the Amaltheidae 

 {Oxynoticeras, Amcdtheus), the Phylloceratidae (Phylloceras), the Lytoceratidae 

 (Lytoceras), and the oldest members of the Stephanoceratidae (Goeloceras, Dactylioccras). 

 It is noteworthy that in the Liassic Ammonites the antisiphonal lobe is frec[uently 

 bifid (as in the Aegoceratidae and Amaltheidae). 



With the exception of the Aegoceratidae all the families that appeared in the 

 Lias lasted into the Middle and Upper Jurassic, although tlie Harpoceratidae are 

 reduced in numbers, and perished in the Malm or Upper Jura. The only new 

 families added in the Middle Jurassic are the Haploceratidae and the Cosmoceratidae. 

 The most common genera in the Middle Jurassic are : Harpoceras, Oppelia, Stephanoceras, 

 Sphaeroceras, Morphoceras, Macrocephalites, Oecoptychius, ReinecJcia, Parkinsonia, 

 Gosmoccras, Perisphindes, Hap)loceras, Phylloceras, Lytoceras. 



In the Malm are found nearly all those genera named under the Middle Jurassic, 

 but the number of species has changed greatly. Thus Harpoceras, Stephanoceras, 

 Reineckia, Parkinsonia and Gosmoceras, are reduced, while Oppelia, Haploceras, 

 Olcostephanus, and especially Perisphinctes have increased greatly. Perisphinctes is 

 decidedly the dominant genus in the Upper Jurassic, and along with it Aspidoceras, 

 Simoceras and PeUoceras show a large number of species. Aberrant forms are rare 

 in the Jurassic, and are confined to a few species of Spiroceras and Baculina. 



A change like that seen at the beginning of the Jurassic takes place also at the 

 end of this period. The Ammonites of the Cretaceous belong largely to new genera. 

 Indeed a remarkable metamorphosis occurs in the entire habitus of the Cephalopod 

 fauna. Only the oldest Neocomian beds of the Alps contain a few species that had 

 lived in the Tithonian epoch, and show the continuity of the two systems. The least 

 degree of change is shown by the Phylloceratidae and the Lytoceratidae. In place of 

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