ZONE OF AEGOCERAS PLANORBIS. 15 



Ammonites planorbis," Wright, ' Quart. Journ. Geol. Soc.,' vol. xvi, p. 389, 1864. " Zone 

 de rAmmonites plaiiorbis," Dumortier, ' :fitudes Paleont.,' t. i, p. 15,1864. "Die 

 Psilonotonscliichten," Braims, 'Die mitere Jm-a,' p. 55,1871. "Zone of Ammonites 

 planorbis," Tate and Blake, Yorkshire Lias, p. 38, 1876. 



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

 general it consists of a series of thin, greyish, or bluish, argillaceous limestones, with 

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

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

 Li the upper half of this group Aec/oceras planorUs (Sow.), in some localities, is found in 

 considerable numbers, compressed in the shales, with the outer layer of its white shell 

 more or less preserved; in the lower portion of the series Ostrea liassica, Strick.,^ 

 appears in great numbers ; and beneath these strata are three or four beds of hard lime- 

 stories (or " firestones "), in which the finest skeletons of Enaliosauria have been dis- 

 covered. As this distinction, by fticans of JE. 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 Amvionites are very rare in the lower 

 beds, and numerous in the upper ; and that Ostre(S are abundant below, and rare 

 above, whilst Saurian remains are found throughout the entire series. 



The best sections oii\ie planorbis-iowe are those afforded by the extensive quarries at 

 Street and the coast-sections at Watchet, and St. Audrey's Slip, in Somerset ;* at Binton 

 and Wilmcote, in Warwickshire; and at Pinhay Bay and Up-Lyme, in Dorset. I 

 purpose giving a detailed description of the most typical sections in each county. 



Somersetshire. — At Street the strata are nearly horizontal and undisturbed, and there- 

 fore admit of accurate measurement. The following section of Mr. Cree's quarry I have 

 compared with like sections afforded by the quarries of Messrs. Seymour, Underwood, 

 and Talbot in the same parish ; and find that the variation is so inconsiderable that any 

 one may be said to represent the others, both as regards the sequence of the beds and the 

 fossils they contain. 



1 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. svi, p. 374, 1864. 



2 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. ill, fig. 1"). Danker, in the ' Palseontographica ' (pi. vi, fig. 27), has figured a small Oyster 

 from the Lias of Halberstadt {Ostrea siMamellosa, 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. 



3 R. Etheridge, F.G.S., "Physical Structure of the Watchet Area," ' Proc. Cotteswold Nat. Field 

 €lub,' 1871, p. 40, and section No. 6. 



