60 



THE LIAS AMMONITES. 



to the subject by remarking " above all, the invariable regularity of the different horizons 

 they occupy." He has further summarised his observations in the following table, which 

 shows how the species are distributed vertically, and how each of the four stages of 

 life is characterised by a certain number of species. 



These observations on Ammonite-zones made by so careful and thoughtful a student 

 of Liassic palaeontology as Dumortier proved himself to be, I commend to the considera- 

 tion of all those who doubt the value of such divisions of the Secondary rocks which I 

 have maintained and followed out in this and other memoirs during the last twenty 

 years. 



Distribution of Jmmonifes in the Zone of Ammonites oxynotus, after Dumortier. 



Beds with 



Beds with 



Beds with 



Beds with 



Am. Davidsoni. 



Am. stbllaris. 



Am. oxynotus. 



Am. raricostatus. 



Am. Davidsoni 



Am. stellaris 



Am. oxynotus 



Am. planicosta 



— resurgens 



— obtusus 



— Victoris 



— raricosiatus 



— Hartmanni 



— • QSdiiensis 



— Aballoensis 



— Cluniacensis 



— Berardi 



— Laiidnoti 



— Buvigneri 



— jejunus 



— Patti 



— Locardi 



— Greenoughi 



— Nodotianus 



— lacunatus 



— Birchii 



— tamariscinus 



— Pellati 



— Savzeamis 



— semicostatvs 



— alius 



— Pauli 



— Scipionianus 



— Boucaultianus 



— Driani 



— armentalis 



— spiratissimus 



— Guihalianus 



— Salisburgensis 



— subplanicosta 







— SfBmanni 



— tardecrescens 







— Bonnardi 



— viticola 







— Dudressiei-i 



— vellicatus 







— Ziphus 



— Edmundi 







— Bodleyi 



— Oosteri 



1 



In this table the two lowest zones have nine, and the two uppermost fourteen, species 

 in each. In the Davidsoni-heds Aeg. Davidsoni and Aeg. Berardi have affinities with 

 Aeg. planorhis ; Arietites Sauzeanus, Ar. Scipionianus, Ar. resitrgens, and Ar. Hartmanni 

 with Ai'ietites bisulcatus ; Ar. spiratissimus with Ar. Conybeari ; and Aeg. lacunatum 

 with Aeg. Charmassei, from which it is derived. 



In the Steliaris-heds Aeg. Q^duensis has affinities with Aeg. Johnstoni ; Aeg. Locardi, 

 with Aeg. BirJiii ; Aeg. Boucaultianum, with Aeg. Charmassei ; Amal. Guibalianus, with 

 Amal. Aballoensis ; whilst Ar. obtusus, Ar. stellaris, and Ar. semicostatus are the expiring 

 forms of the genus Arietites. 



The Oxynotus-hQd^ are characterised by new types of Ammonite life belonging to 

 the genus Amaltheus, which now appear for the first time as Amal. oxynotus, Amal. Victoris, 

 Amal. Aballoensis, Amal. Greenoughi, Amal. Buvigneri, smd Aeg. planicosta ^=^ ziphus ^=- 

 Dudressieri, which last are morphological conditions of one species Aeg. planicosta. 



The Baricostatus-heds are characterised by forms which depart from the Arietites type ; 

 they have a wide umbilicus, narrow whorls, and were slowly developed as Ar. Nodotianus, 

 Ar. viticola, Ar. vellicatus, Ar. Edmundi, which are all nearly allied to Ar. raricostatus. 



