26o JURASSIC. 



where some conglomeratic modifications are met with. On the 

 Dorset coast it is upwards of 450 feet, and in Yorkshire 750 feet. 



The study of the Ammonites has led palaeontologists to make 

 certain subgenera, which may be useful to specialists, but which 

 are not, as a rule, welcomed by the field-geologist :^ hence the 

 Zones of the Lower Lias may be noted as follows : — 



Ammonites (Al^goceras) capricornus, and A. {ALgoceras) Henleyi. 



,, (Amaltheiis) ibex, and A. {yEgoeeras) Jamesoni. 



,, (yEgoeeras) antiatus and A. {Arietites) raricostatus. 



,, {Amalt/ietis) oxynotiis. 



,. {Arietites) obtiisiis. 



,, {Arietites) Tnrncri, and A. {Arietites) semicosiatus. 



,, {Arietites) Bitcklaiidi. 



,, {ALgoceras) angitlatns. 



, , {Ai.goceras) plaiiorbis. 



The student must not be disappointed if in any one tract of country he fails to 

 identify all the above zones. Many of them may be made out with time and 

 patience in the cliffs of Lyme Regis and Charmouth, but even tliere he would 

 be a bold man who stated pi'ecisely where one zone ended and another began. 

 The general succession of forms is, however, an interesting fact, although here and 

 there two or more of the zonal species may be commingled. 



The zones in the Lower Lias of this country were first marked 

 out by Dr. Oppel, and were subsequently examined in more detail 

 by Dr. Wright.^ Broadly speaking, the stone-beds comprise the 

 zones up to that of A. BnckJandi and A. semicosiatus; the clays 

 comprise the zones above. Some of the more abundant fossils 

 may now be mentioned. 



In the zone of A. planorbis (argillaceous limestones and shales) 

 — Unkardmm cardioides, Ostrea Liassica, Peden Pollux, Avicula 

 cygnipes, Modiola minma, Lima punctata, L. gigantea (small speci- 

 mens), Pleuromya Crocombeia, Ammonites Johfistoni {torus), Cidaris 

 Edivardsii, Hemipedina Tomesii, etc. 



In the zone of A. angulatus (limestones, etc.) — Lima punctata, 

 L. gigantea, I\Lvoconcha psilonoti. Pinna Hartmanni, Cardinia ovalis, 

 Unicardium cardioidcs, Nautilus striatus, Ammonites Conybeari, A. 

 Charmassei. 



In the zone of A. Bucklandi (argillaceous limestones and clays) 

 — Rhynchonella variabilis, Gryphaa incurva (or aixuata, known also 

 as the Devil's Toe-nail), Modiola Hillana, Cardinia Listen, Lima 

 gigantea (large specimens), Pholadomya glabra, Spirifera Walcottii, 

 Nautilus stfiatus, Ammo7iites Conybeari, Pcntacrinus tuberculatus 

 (Star-stones), etc. 



In the zone of A. Turneri (pale earthy and shelly limestone 

 and clay) — Rhynchonella vajiabilis, Cardinia ovalis, Gervillia lanceo- 

 lata, Avicula imxquivalvis, Ammonites semicosiatus, etc. 



In the zone of A. obtusus (clay and marl with inconstant bands 

 of limestone) — Pleurotomaria Anglica, Ammonites tnul/icostatus, A. 

 Brooki, A. Birchii, A. stellaris, A. planicosta, Extracrinus Briareus, etc. 



1 See R. Etheridge, Address to Geol. Soc. 1882. 



2 Q. J. xvi. 411. See also Stratigraphical and Physical Geology, by R. 

 Etheridge, 1885. 



