54 



LOWER LIAS. 



. The above table clearly shows how completely the whole Lias formation is represented 

 in England, and how nearly it corresponds bed for bed with the Wurtemberg series. I 

 shall now describe the different zones in ascending order, commencing with the lowest, the 

 zone of Ammonites jplanor lis. 



THE LOWER LIAS. 



1. The Zone of Ammonites planorbis. 



Synonyms. — "White Lias," William Smith, 'Memoir to the Map/ p. 47, 1815. 

 " White Lias (pars)," De la Beche, ' Geol. Trans.,' 2nd series, vol. ii, p. 26. " Saurian 

 Beds," Murchison's ' Geology of Cheltenham,' 2nd ed., by Buckman and Strickland, 

 p. 49, 1845. " Psilonotenbauk," Quenstedt, ' Der Jura,' Table, p. 293, 1857. "Die 

 Schichten des Ammonites planorbis," Oppel, ' Juraformation.' p. 24, 1856. "Zone of 

 Ammonites planorbis," Wright, ' Quart. Jour. Geol. Soc.,' vol. xvi, p. 389. 



This division of the Lower Lias is well developed in the South of England.^ In 

 general it consists of a series of thin, grayish, or bluish, argillaceous limestones, with alter- 

 nating beds of laminated shale ; or sometimes it forms the upper part of the thick-bedded 

 argillaceous, cream-coloured limestone, called " White Lias " by William Smith. In 

 the upper half of this group of beds Ammonites planorbis, Sow., in some localities is found 

 in considerable numbers, compressed in the shales, with its white shell more or less 

 preserved ; in the lower portion of the series Ostrea liassica, Strickl.,^ appears in great 

 numbers; beneath these strata are three or four beds of hard limestones (or "fire- 

 stones "), in which the finest skeletons of Enaliosauria have been discovered. As this 

 distinction, by means of Am. planorbis, Ostrea liassica, and Saurians, is a practical and 

 useful one in the investigation of this zone of life, I shall adhere to it on the present 

 occasion, — premising, however, that Ammonites are very rare in the lower beds, although 

 abundant in the upper ; and that Ostrece are abundant below, but rare above, whilst Saurian 

 remains are found throughout the entire series. 



The best sections of the zone of Ammonites planorbis are those aff'orded by the 

 extensive quarries at Street and the coast section at Watchet, in Somerset ; at Binton 



^ The substance of the following observations on the Lower Lias is contained in my memoir on the 

 "Zone of Avicula contorta, and the Lower Lias of the South of England," 'Quart. Jour, of the 

 Geological Society,' vol. xvi, p. 374, 1860. 



" Ostrea liassica, Strickland, is a very characteristic shell of the lowest Lias beds. It resembles 

 Ostrea irregularis, Miinster (Goldfuss, ' Petr. Germ.,' pi. vii — ix, fig. 5), and Ostrea rugata, Quenstedt 

 ('Jura,' pi. iii, fig. 17). Danker, in the ' Palseontographica,' (pi. vi. fig. 27), has figured a small Oyster 

 from the Lias of Halberstadt {Ostrea sublameUosa, Dunker), which appears to be identical with our species. 

 These small, thin, rugose Oysters are found in great abundance in the lowest beds of the Lower Lias in 

 England and Germany. They are probably only varieties of one species, which had a wide geographical 

 distribution in the seas which deposited the basement-beds of the Lias. 



