Middle L ias. 1 69 



of which is formed of a tough brown fossiliferous 

 limestone, generally of only a few feet in thickness, but 

 nearly constant in its occurrence from Dorsetshire to 

 Yorkshire, and the very indefinite base of the Marlstone 

 forms the eastern boundary of the Lower Lias. 



The Lower Lias clay and limestone of England is, as 

 a whole, rich in fossils, the general grouping of which 

 cannot be more than noticed here in a cursory manner. 

 These strata yield Extracrinus among the Crinoids, 

 (fig. 35); of Brachiopoda, a few species of Spiriferce, 

 Terebratulce, and Rhynclionellce, and numbers of 

 Lamellibranchiate molluscs, such as Gryphcea incurva, 

 Oysters, Pectens, Limas, Pinnas, Aviculas, Pholado- 

 myas, and others. Having been deposited mostly in 

 deep sea, univalve shells are much less common, but of 

 the Cephalopoda, which are free swimmers, there are 

 vast numbers of Ammonites, Belemnites, and Nautili 

 (see fig. 35), together with many fishes, and the great 

 marine Enalosaurian reptiles, Ichthyosaurus (fish- lizard) 

 and Plesiosaurus (see fig. 36), and the insectivorous 

 flying reptile, Pterodactylus brevirostris. 



THE MAKLSTONE SERIES, or MIDDLE LIAS, which suc- 

 ceeds the Lower Lias clay, is generally somewhat argil- 

 laceous below, graduating upward into a brown, fer- 

 ruginous, soft, sandy rock, with hard nodular bands, and 

 a very marked brown ferruginous limestone at the top. 

 It is ricn in many forms of Ammonite, Belemnite, 

 Plagiostoma, Lima, Pinna, Pholadomya, Pecten, 

 Modiola, Terebratula, and Rhynchonella, besides a 

 very characteristic Spirifer (S. Walcotti, fig. 36), one 

 of the few remaining shells of that Palaeozoic genus. 

 Where the Lower Lias and Marlstone join, the strata 

 graduate into each other, but through the central parts 

 of England these passage-beds are rarely clearly ex- 



