78 
CASH: FOSSIL MOLLUSCA. 
the Carboniferous area, and the wide vales of the Permian and Trias 
to the wonderful development of Jurassic and Cretaceous rocks 
on the east coat, with patches here and there of Post Tertiary 
beds, — and Fossil shells are to be found in most of these forma- 
tions, in many instances in immense profusion ; their value is 
g-reatly enhanced to the field g-eolog-ist when they are found to 
be characteristic of particular strata and beds, this is found 
often to be the case, and this most notably in the so-called 
Ammonite Zones in the Lias, where the presence of one single 
Ammonite may be sufficient to indicate the exact stratigraphical 
position of a Geological stratum. 
The following genera of fossil shells have been recorded for 
the county of York in Tate and Blake's Yorkshire Lias ; 
Phillip's Geology of the Yorkshire Coast; Phillip's Geology of the 
Mountain Limestone District; Davis and Lee's West Yorkshire ; 
the volumes of the Quarterly Journal of the Geological Society, &c. 
No doubt some of these genera will be found on a careful 
study of specimens to be merely synonyms, others as the 
Ammonites, the Goniatites, some of the Lamellibranchs will require 
to be split up, at least into sub-genera. 
GENERA OF YORKSHIRE FOSSIL CEPHALOPODA. 
Tenth opsis 
Beloteuthis 
Geoteuthis 
Belemnites 
Belemnitella 
Ammonites 
Amaltheus 
Lytoceras 
Phylloceras 
Harpoceras 
Stephanoceras 
CEgoceras 
Arietites 
Cosmoceras,&c. 
Scaphites 
Hamites 
Jurassic 
Jurassic and Cretaceous. 
(Cretaceous 
