386 STEATIGRAPHICAL GEOLOGY 



*Cryptsenia, Cylindrites, Dicroloma ( = Alaria), *Exelissa, Littorina, 

 Monodonta, Nerita, Neritopsis, Patella, Pseuclomelania, *Purpurina, 

 Solarium, Scalaria, *Trochotoma, Turritella. 



Cephalopoda. *^Egoceras, *Amaltheus, *Arietites, *Ccelo- 

 ceras, *Dactylioceras, *Deroceras, Harpoceras, *Haugia, *Hildo- 

 ceras, *Grammoceras, *Liparoceras, Lytoceras, Holcostephanus, 

 *0phioceras. *0xynoticeras, *Paltopleuroceras, *Phylloceras, 

 *Psiloceras, *Schlotlieimia, Belemnites. 



Pisces. Acrodus, *Dapedius, *Eugnathus, Hybodus, Lepi- 

 dotus, *Leptolepis, *Pholidophorus. 



Beptilia. *Dimorphodon (a Pterodactyle), *Scelidosaurus (a 

 Dinosaur), *Steneosaurus and *Pelagosaurus (Crocodiles), *Plio- 

 saurus, Ichthyosaurus, Plesiosaurus, and Eretmosaurus (the last 

 three, however, are represented in the Rhsetic bone beds). 



I. THE LIAS OR LOWER JURASSIC SERIES 



A. THE BRITISH LIAS 



Subdivisions. This series has on the whole a very uniform 

 lithological aspect throughout England. It consists almost every- 

 where of four successive sets of strata, and might with good reason 

 be divided into four stages instead of the three which are usually 

 adopted in England, for such a subdivision is not inconsistent with 

 the range of the fossils, and is actually in use on the Continent (see 

 table below). Thus (1) its lowest beds are more or less calcareous, 

 and consist generally of thin-bedded limestones and shaly clays ; 

 (2) above these comes a thick mass of dark-grey clays ; (3) these 

 are succeeded by sandy clays and sandy limestone (marlstone) ; 

 (4) while at the top are thin limestones and marly clays. 



A further subdivision of the Lias into zones has resulted from a 

 careful study of the distribution of its fossila These zones are each 

 characterised by one or more species of Ammonites, and though 

 some of them are more or less local, and cannot be traced all across 

 England, they can be grouped into nine principal zones which are 

 so traceable. The following table shows the divisions in general 

 use, the principal zones, and the continental divisions : 



Lithology. Zones. Cont. Bquiv. 



t Grammoceras aalense "\ 



I Dumortieria radians 



Upper / Clays and I Lillia Lilli [ Toarcian 



Lias \thin limestones j Grammoceras striatulum I 

 Dactylioceras commune 

 Harpoceras serpentinum J 



