MIDDLE LIAS. 2/1 



A?)imonitcs spinatiis characterizes the Rock-bed ; and the other 

 species the lower micaceous sands and clays. But the two forms 

 occur together in the Rock-bed at Ihiiinster and near Yeovil. 



By some authorities the lower zones of A. capriconius, A. ibex, 

 and A. Hcnhyi have been included with the Middle Lias. 

 (See p. 258.) Among the fossils are A. Engelhardti, A. fimhriatus, 

 Belemnites compressus, B. hreviformis, Protocardium truncatiwi, 

 Hinnitcs ahjcctiis, Pccteii ccquivalvis, ISIyacites unionides, Pholadoinya 

 ambigua, Modiola scalprtnn, Gryphcva cymbium {Macctillochii or 

 obliquata), Terebratiila punctata, Waldhcimia quadrifida, var. cornuta, 

 Rhynichonella tetrahcdra, R. acuta, etc. 



The Lower Lias passes upwards into the Middle Lias by 

 insensible gradation, and as the boundary-line between the two 

 formations is essentially a palseontological one, there is no marked 

 division. It is indeed one of the most vague boundary-lines that 

 is drawn upon geological maps. 



The fossils are decidedly most numerous in the Marlstone. 

 This bed at Ilminster has yielded many Plant-remains, Fora- 

 minifera, Echinodermata, Crustacea, Brachiopoda, Conchifera, 

 Gasteropoda, Cephalopoda, bones of Ichthyosaurus, teeth of 

 Hybodus, and scales of Lepidotus} 



On the Dorsetshire coast the Middle Lias is exposed in the cliffs 

 between Charmouth and Bridport Harbour. (See Fig. 40, p. 252.) 

 Curiously enough the upper part of the Middle Lias is united in 

 one bed with the base of the Upper Lias, as observed by Mr. 

 Etheridge in 1861. This is a pale cream-coloured and pink lime- 

 stone in the upper part, and a brown nodular marlstone below. 

 Amvionitcs bifrons and A. serpent inus occur in the upper part of the 

 bed, and A. communis occurs in both. A. spinatus is found in the 

 lower part, together with several species of Pleuroiomaria. The 

 following is the general succession of the beds :^ — 



Upper Lias stone ) ^ 



/-Marlstone \ 3 leet. 



Clay 10 to 20 ,, 



Yellow micaceous sands, with indurated beds 60 ,, 



Bluish-grey marl [Afargaritatus-h^d of Mr. Day at base) 7 ,, 



Laminated sands and clays, with many layers of sandy 

 nodules in the upper part, and beds of irregular and 



fissile sandstone in the lower 70 to 80 ,, 



Starfish-bed 5 ,, 



Grey micaceous clays and marls, with A . via7-garitaltis, etc. 1 60 , , 



I The Three Tiers. (See p. 264.) 35 ,, 



370 ,, 



Ammonites margaritatits has been met with in the Three Tiers, and it is found in 

 some abundance in the grey marly clays above, although here, as a rule, only frag- 

 ments are to be obtained. The Starfish-bed is a greenish-grey micaceous and 

 slightly calcareous sandstone, upon the flat under-surfaces of which are found 

 Ophioderma Egertoiii and O. tcniiibi-achiata. This bed and the beds above are 



^ C. Moore, Proc. Somerset Arch. Soc. xiii. 

 * E. C. H. Day, Q. J. xix. 286. 



Middle 

 Lias. <( 



