GEOLOGICAL DISTRIBUTION OF CEPHALOPODA. 207 



modified descendants of the nautiloids, a transition to which ap- 

 pears to have been effected by way of the genus Goniatitcs and 

 those forms of the Carboniferous period (India and Texas) which, 

 like Arcestes, have the sutural plication intermediate between what 

 is seen in Goniatites (Silurian-Permian) and Ceratites (Triassic). 

 Just where the embranchment from the nautiloid line took place 

 it has been impossible to determine, but it is significant that the 

 most nautiloid form of the Ammonitidse, the Goniatites, appeared 

 after the Nautilidse had attained their maximum development, and 

 some time after the genus Nautilus had itself appeared. Sutner 

 estimates that there are in the neighbourhood of four thousand 

 species belonging to the group of the ammonoids, nearly all of 

 which have the foliated sutures characteristic of the true ammo- 

 nites. With the exception of the genera Goniatites and Clymenia, 

 and the primitive ammonitic forms of the Carboniferous rocks 

 already referred to (Sageceras, Arcestes, Xenodiscus, Medlicottia, 

 Cyclolobus *), and a single form which occurs in the lowest member 

 of the Californian Tertiaries (the Tejon group), all the species are 

 restricted to the Mesozoic deposits, which by their great numerical 

 development they might be said to characterise. Probably no other 

 group of invertebrates exhibits such a remarkable series of devel- 

 opments corresponding with successive periods of time as do the 

 Ammonitidse, and in none do the species appear to be so distinc- 

 tively characteristic of certain horizons (zones of ammonites). The 

 singularly diversified types of the Triassic period, which combine 

 all the various sutural modification seen in the goniatitic stage 

 (Sageceras, Lobites), the ceratitic (Tyrolites, Celtites), and the 

 ammonitic, from the simplest to the most complex (Pinacoceras), 

 are almost wholly wanting in the Lias, where an entirely new series 

 of forms begins (^Egoceras, Harpoceras, Amaltheus). These in turn 

 are succeeded by groups more or less distinctive of the different 

 Jurassic zones (Oppelia, Stephanoceras, Lytoceras, Phylloceras), 

 which in the main die out before the close of the period. Most of 

 the Cretaceous forms belong to genera or sub-genera which have not 

 hitherto been represented, and here for the first time do we find any 

 great development of the remarkable groups of uncoiled ammonites 

 Scaphites, Hamites, Turrilites, Crioceras, Ancyloceras (also Juras- 

 sic), and Baculites whose advent seems to be foreshadowed by the 



^ Cyclolobus Las the typical ammonitic sutures, 



