UPPER LIAS: PASSAGE BEDS. 253 
Grossly a donaciformuf. 
Inoceramus cinctus. 
- dubius (Fig. 80). 
Leda ovum (Fig. 83). 
Nucula claviformis. 
Hammeri. 
Pecten pumilus. 
Pleuromya costata. 
, Thracia glabra. 
Trigonia pulchella. 
Discina reflexa. 
Serpula tricristata. . 
Pentacrinus jurensis 
The zone of Ammonites jurensis, together with the overlying 
zone of A. opalinus, are in parts of the west of England associated 
for stratigraphical purposes, under the name of the Midford Sands. 
Such a division may be looked upon as forming a connecting link 
between the Lias and the Oolites, constituting as it does passage- 
beds from the one group to the other. Under such circumstances 
it matters little whether we treat the Midford Sands under the 
heading of Lias or Oolites, and as a matter of convenience we 
shall note the details with the newer formation, for they cannot 
well be treated separately, on account of their intimate stratigra- 
phical association. 
On general grounds it is thought desirable to draw the line 
between Lias and Oolites in the midst of this series of passage- 
beds, and most authorities agree in placing the zone of A. jurensis 
with the Upper Lias, and that of A. opalinus with the Inferior 
Oolite. This divisional plane, such as it is, is here adopted on 
palseontological grounds, although it must be admitted that on 
stratigraphical grounds the general division of Midford Sands 
is the most important one to represent on the Geological Survey 
Maps in the country from Dorsetshire to Gloucesterskire. That 
zones transgress the limits of stratigraphical divisions is admitted; 
and the researches of Mr. S. S. Buekman (to which more attention 
will be drawn in our next volume) show that in Dorsetshire the 
/one of A. jurensis extends below the stratigraphical limits of the 
Midford (or Bridport) Sands. Moreover in the Midland counties 
the Northampton Sands, which include the zone of Ammonites 
opalinus, include also a portion, at any rate, of the zone of 
A. jurensis. 
Notwithstanding the separation of these strata into the zones of 
A. jurensis and A. opalinus, there is no unanimity amongst geolo- 
gists on the precise palaeontological division between these zones, 
for other minor zones have been introduced, and the more atten- 
tively the beds are studied the less reason do we find for fixing 
any special plane of division, where the sequence is complete. 
