88 HANDBOOK OF FOSSILS. 



of the huge fish-lizard (Ichthyosaurus t p. 24), and long-necked 

 Plesiosaurus (p. 25), besides numberless fish ; whilst the latter 

 is renowned for its jet (or fossilized wood) and 

 its "snake-stones" (Ammonites)^ concerning 

 which curious old stories are told. Ammonites 

 are plentiful in the Lias, which has been sub- 

 divided into zones, or layers, named after the 

 ammonite occurring in greatest numbers in that 

 particular zone. There is one thin limestone 

 band in the Marlstone composed entirely of the 

 shells of Ammonites planicostatus. A curious 

 kind of oyster (Gryphaa incttrva), locally known 

 as the devil's toenail, a huge Lima (L. gigantca), 

 a magnificent Encrinite (Extracrinus Briarens), 

 and numerous other fossils, are also to be ob- 

 tained by patient search. 



5. Rhoetic, Penarth Beds, or "White Lias. 



These beds are not of any considerable thick- 

 ness, but are very persistent, and of great in- 

 terest, inasmuch as they yield the remains of 

 the oldest known mammal (Microlestes), a small 

 insect-feeder. They are composed of limestones, 

 shales and marls (i.e. limey clays), and are best 

 studied in Somersetshire and Dorsetshire. The 

 "landscape marble" belongs to this formation, 

 which also contains a bone bed, or thin layer 

 made up of the bones and teeth, etc., of fish. 

 Shells are not numerous, though the casts of 

 one species (Avicttla contorta] is plentiful. 



6. Trias, or New Red Sandstone, a thick 

 series of sandstones and marls, the great mass 

 of which forms the subsoil of the western mid- 

 land counties, Birmingham being nearly in the 

 centre, thence they extend in three directions, 

 one branch passing towards the north-west, 

 through Cheshire, to the sea at Liverpool, re- 

 appearing on the coast line of Lancashire, West- 

 moreland, and Cumberland, where it also forms 

 the Valley of the Eden. Another branch extends 



Behmnitas elon- through Derby and York to South Shields, 

 lias)!" ' IC whilst the third ma y be traced southwards in 



isolated patches down into Devonshire. 



There are scarcely any fossils in it, but in Worcestershire and 

 Warwickshire the bivalve shell of a small crustacean (Estheria 



