J. C. Mansel-P ley dell — Geology of Dorset. 403 



British Association in 1864, as everywhere underlying the true 

 Liassic strata. They consist chiefly of a central mass of black shale 

 resting on, and passing into, the underlying Keuper marls ; whilst 

 their upper division, which forms the base of the Lias, characterized 

 by the presence of Ammonites planorbis, and Ostrea liassica, is com- 

 posed of beds of marl and marly or argillaceous limestone. Their 

 upper limit is well marked by the White Lias, which, in Mr. Bristow's 

 opinion, forms its uppermost member. The number of fossils from 

 these beds is considerable, comprising thirty-two genera and fifty-one 

 species of shells, six genera and seven species of fish, two reptiles, 

 and one mammal. Of these Monotis (Avicula) decussata is the only 

 species common to this and the overlying Liassic formation, although 

 there is no marked lithological break nor observable unconformity. 



LiA.s. — The Lower Lias, which is extensively shown at Lj^me 

 Eegis, has been, until recently, massed as an indivisible series of 

 clay, shales, and limestones. Paleontologists have now divided it 

 into six zones, each of which is particularly marked, by its own 

 group of fossils, especially by Ammonites: but this grouping has 

 not been found to hold good over large areas. From Devonshire to 

 Yorkshire the divisions, Upper, Middle, and Lower, are said to be 

 generally constant, and no actual unconformity is known from the 

 bottom of the Lower to the top of the Upper Lias. The zones thus 

 adopted in an ascending order are, 1. Ammonites planorbis zone ; 

 2. Ammonites Bucklandi ; 3. Ammonites Turneri; 4. Ammonites obtusiis ; 

 5. Ammonites oxynotus; 6. Ammonites raricostatus. The Ammonites 

 BucMandi zone attains a thickness of more than eighty feet at Lyme, 

 consisting of grey limestones, alternating with marls and shales ; 

 its lower division is characterized by Ammonites angulatus, and con- 

 tains four species of Ichthyosauri. The Ammonites Turneri zone is 

 characterized by several species of Cephalopods ; an abundance of 

 the Crinoid Pentacrinus tuberculatus, and the Saurians Ichthyosaurus 

 platyodon, I. communis, and /. intermedius. The Ammonites obtusus 

 zone contains many species which appear for the first time, among 

 which are Ammonites planicostatus, A. Birchii, A. BrooJcii, A. Diidres- 

 sieri, Nautilus striatus, and Belemnites acutus. Exiracrinus briareus 

 belongs to the limestone beds of this zone. The Ammonites raricos- 

 tatus zone yields several Cephalopods of that family, namely, 

 Ammonites armatus, A. nodulosus, A. Guibaldianus ; it forms part of 

 the Hippopodium bed of Murchison. The beds between this and the 

 Ammonites spinatus zone inclusive comprise the Middle Lias, which 

 at Lyme is upwards of five hundred feet thick ; this increased 

 development, as compared with the other British representatives of 

 the Middle Lias, occurs, as might be expected, at a point in close 

 proximity to its continental equivalent. At Black Veu and Golden 

 Cap its lowest beds consist of grey marls, capped in the last-named 

 locality by sand, containing nodules, wliich appear within twenty- 

 five feet of the beach-line. At Down Cliff, where a fault lets down 

 the marls, and hides them from view, the downthrow is eastward, 

 about two hundred feet. The Star-fish bed, so called from the 

 Ophioderma Eyertoni, belongs to this section, also a band yielding 



