272 LIAS OF ENGLAND AND WALES : 
The " Communis " Beds consist of grey and blue marly and 
shaly clay, 2 to 8 feet thick, surmounted by flaggy and nodular 
limestone bands (" Upper Cephalopoda Bed ") about 1 ft. 6 inches 
in thickness. The stone is sometimes oolitic, according to Mr. 
Thompson. He records Ammonites bifrons, A. communis (very 
abundant), A. cornucopia, A. Holandrei (abundant), A. scmicelatus, 
A. serpentinus (falcifer), Belemnitcs canaliculatus , B. irreoularu, 
Trochus duplicatus, Nucula Hammeri, N. claviformis, &c. 
It will bs noticed that the name " Communis Beds" is applied, 
locally, to the lower part of the zone of A. communis ; neither 
palseontologically nor stratigraphically is this division distinctly 
separable from the Basement Beds of other localities, but as it is 
considered to be somewhat local, i( may be taken as an indication 
of the fact elsewhere noted, that the Basement Beds shade np into 
the overlying strata. (See p. 248.) 
The upper part and indeed the main mass of the Upper Lias in 
this, as in most other tracts, consists of blue clay with nodules of 
limestone and septaria. 
In his detailed notes on the Upper Lias of Northamptonshire 
Mr. Beeby Thompson has subdivided the clays as follows : 
("Upper Leila-ovum Beds = " Jurensis " Zone (in part). 
T p, j Middle Leda-ovum Beds. 
Upper Lias .lay - Lower Le da-ovum Beds or Cerithium Beds. 
I Unfossiliferous Beds. 
Among the more abundant fossils that have a wide range, arc 
Ammonites bifrons, A. cornucopia, A. heterophyllus, A. serpentinus 
(falcifer'), Area elegans, Enomphalns (?) minutus, Inoceranuix 
dubius, and Lcda ovum. Bclemnites occur more or less abundantly. 
The total thickness of the Upper Lias in the boring on the 
Kettering road near Northampton, was 153 feet ; and at Dustou, 
190 feet* 
The " Unfossiliferous beds" consist of dark blue pyritic clay, 60 feet and 
upwards in thickness. Selenite is abundant, and jet is also found. 
Calcareous spar exhibiting cone-in-cone structure and known as " Nail- 
head Spar " is also found in the upper part of the beds.f Occasionally fossils 
are obtained, and Mr. Thompson records Ammonites bifrons, A. communis, A. 
Holandrei, A. serpentinus, Cerithium, Inoceramus dubius, and Nucula Hammeri. 
The occurrence of such Unfossiliferous beds, accompanied by selenite, may 
be attributed simply to the destruction of the organic remains and the con- 
sequent formation of selenite. Nevertheless locally the beds seem to occur 
along a fairly persistent horizon. 
The Lower Leda-ovum beds are distinguished by the abundance of small 
Gasteropods, especially Cerithium armatum, and by the prevalence of Ammo 
nites fibulatus and Nucula Hammeri. Among other species recorded by Mr. 
Thompson, are Ammonites bifrons, A. crassus, A. Holandrei, Nautilus, Acttro- 
nina, Euomphalus(?) minutus, Area elegans, Dentalium liassicum, Inoceramus 
dubius, Leda ovum, Pecten pumilus, Discina reflexa, &c. 
No distinguishing fossils are noted from the Middle Leda-ovum Beds, 
which in fact form but a passage from the lower to the higher stages of this 
clay-division, and do not constitute a group of any special value. 
* H. J. Eunsoii, Quart. Jouru. Geol. Soc., vol. xl. p. 486 ; B. Thompson, Middle 
Lias of Northamptonshire, p. 101. 
f This firusture occurs also in the Upper Lias of Yorkshire. See p. 277, and 
ways, Jurassic Rocks of Yorkshire, pp. 127, -TSfi. 
