246 LIAS OF ENGLAND AND WALES : 
shire 176 feet, in South Lincolnshire 200 feet, near Lincoln itself 
100 feet, and further north in that county the thickness diminishes 
to 70, and probably to as little as 25 feet. 
The clays that constitute the mass of the formation, shade up 
irregularly and gradually into the Sands at the base of the 
Inferior Oolite, from Dorsetshire to the Cotteswold Hills, so that 
here we have no definite stratigraphical division ; but further 
north in Oxfordshire, Northamptonshire, and Lincolnshire, there 
is usually a marked divisional plane between the Upper Lias clay 
and overlying beds (Northampton Sands), accompanied here and 
there by the evidences of local erosion. 
The Upper Lias has been subdivided into several zones, but 
somewhat differently by various geologists,* and this is not to be 
wondered at considering that the so-called zonal species, Ammo- 
nites annulatus, A. serpentinus (or falcifer\ A. communis, and A. 
bifrons, occur together in the Basement Beds of the division. As 
a matter of general convenience, and having regard to the preva- 
lence of certain species at particular horizons and to the associated 
fossils, the zones have been arranged in descending order as 
follows : 
Zones. 
{Sands and clays (Midford "1 4 .. 
c< j / * j XT . Ammonites rurensis 
Sands in part, and North- > /p. ^ 
ampton Sands in part). J 
Clays (main mass"! ni j cv, i 1 Ammonites communis 
c TT T \ r Ways and onales - - > /-p- ~.> N 
of Upper Lias). J J j (vig. 73). 
f~ Limestones, clays, and paper- "| Ammonites serpentinus 
,, , u _, shales (Fisli and Insect I (Fig. 70) (or A.fal- 
Limestones,LeptamaBeds, f cifer) and A. annu- 
[_ and Transition Bed). j latus. 
The "Transition Bed," which is a fossiliferous layer a few 
inches thick, connecting the Middle and Upper Lias, has been for 
convenience described with the Middle Lias. 
It may be mentioned that Ammonites bifrons can no longer be 
regarded as a zonal species, as it has far too wide a range, occur- 
ing throughout the Upper Lias. Dumortier indeed has treated 
the whole of the Upper Lias (below the zone of A. opalinus], as the 
zone of A. bifrons. 
Objections might locally be urged against other zonal species, 
such as A. striatulus ,t which occurs in the Basement Beds of the 
Upper Lias in Dorsetshire, while it has been recorded from the 
Fish and Insect Limestone of Byfield (on the authority of Dr. 
Wright). From the beds on the same horizon at Navenby in Lin- 
colnshire, the species has been recorded (with a query) by Messrs. 
Sharman and Newton. J 
* Wright, Lias Ammonites, pp. G7, 116, 163 ; Judd, Geol. Rutland, p. 89 ; 8. S. 
Buckman, Inf. Ool. Ammonites, p. 114, and Quart. Joura. Geol. Soc., vol. xlv. 
p. 440 ; B. Thompson, Kep. Brit. Assoc. for 1891, p. 334. 
t This is probably the form recognized as A. radians by Day, Quart. Journ. 
Geol. Soc., vol. xix. p. 295. See also Judd, Geol. Rutland, pp. 82, &c. ; F. Smithe 
and W. C. Lucy, Proc. Cotteswold Club, vol. x. p. 206. 
J Ussher, Geol. Lincoln, p. 180. 
