100 



PALAEONTOLOGY. 



ginal when young, but gradually becoming more central in 

 the adult; it has no special relation to Clymenia. 



Orthoceratidce. — The simplest form of Orthoceras is like 

 a Nautilus unrolled; and Liiuites (fig. 30, 2) is the same 

 with the apex spiral. Those species of Orthocerata in which 

 the aperture is contracted, form the genus Apioceras, Fischer 

 ( = Poterioceras, M'C), or when also curved, the Oncoceras of 



Fig. 30. 



1. Nautiloceras Omalii, de Kon. ; Carboniferous, Belgium. 



2. Lituites (Breynius) ; U. Silurian, Sweden. 



3. Section of Clymenia, shewing internal siphuncle ; Devonian, Petherwin. 



4. Section of Camaroceras duplex, Wakl. ; L. Silurian, Russia. 



5. Siphuncle of Huronia Bigsbyi, Stokes ; with outline of shell and septa. 



6. Siphuncle of Discosorus, Hall; U. Silurian, Lake Huron. 



7. Phragmoceras ventricosum, Sby. ; L. Ludlow rock, Herefordshire. 



8. Gyroceras Eifeliense, d'Arch. ; Devonian, Prussia. 



9. Ascoceras Bohemicum, Barr. ; U. Silurian, Prague. 

 10. Goniatites, Henslowi, Sby.; Carboniferous, Asturias. 



Hall. In Barrande's genus Ascoceras (fig. 30, 9), the shell is 

 flask-shaped, the chambered and siphunculated apex being 

 apparently deciduous; the aperture is contracted, and the 

 air-chambers occupy only the dorsal half of the shell. In 

 Phragmoceras (fig. 30, 7), the shell is slightly curved to the 

 ventral side, and the aperture is remarkably contracted, the 



