MIDFORD SAND. 41 
Beds, but it is not always practicable to accommodate our strati- 
graphical subdivisions strictly to palasontological horizons, even if 
we could find fossils more plentifully than AVC do, and were 
assured that the species were confined within definite limits. We 
know, however, that index-species are not always confined to the 
zones they indicate ; they are considered only to be dominant at 
particular horizons, and this information rests largely on local 
evidence. 
Whatever view we take of the species or " mutations " of 
Ammonites, and of the value of minute divisions of the strata, 
there is no means by which we can fix a boundary in this tran- 
sitional series of strata in the west of England that can be traced 
with confidence, or that would have any practical value. 
Considering the vexed subject of species, it is of course difficult 
to enumerate the particular fossils that may be said to belong 
especially to each of the two zones of A. jurensis and A. opalinus. 
Nor are we helped out of the difficulty by reference to the fossils 
recorded from continental strata, for as regards the species that 
have been taken as a basis for classification, we find a varying 
range assigned to them in different localities. For instance, such 
forms as A. aalensis, A. hircinus, and A. subinsignis are recorded 
from both zones. 
In Yorkshire the name " Striatidus-beds " is applied generally to the 
zone of A. jurensis, and the evidence obtained in other parts of England 
tends to show that the species may have appeared in places much earlier, 
for we find it in the Basement Beds of the Upper Lias in Dorsetshire. In 
the Gloucestershire Cephalopoda Bed, A. radians and A. striatulus are in- 
timately associated, and the species are very closely allied, f Mr. S. S. 
Buckman makes use of these two species to mark distinct sub-zones, both 
of which he places in the Upper Lias (zone of .4. jurensis). Mr. Hudleston 
places the "Radians-zone" at the base of his Lower division of the In- 
ferior Oolite. A. aalensis is recorded by Tate and Blake from beds low 
down in the Upper Lias of Yorkshire, while it occurs with A. opalinus in 
the Gloucestershire Cephalopoda Bed, and with A. jurensis on the Dorset 
coast. A. opalinus again is recorded 'from Dorsetshire in the same bed 
with A. Murcliisonce.^ 
Such occurrences, which are not altogether dependent on varying views 
of species, are natural enough, and are parallelled by what we know of the 
distribution of species in the Lias. They indicate the varying local range 
of different species, and they show that we cannot rely on the occurrence 
of one or two specimens of a species to fix a positire stratigraphical 
horizon. 
In the following list I have recorded the species that are said 
to characterize particular zones ; but for geological purposes it is 
convenient to take the fauna as a whole, regarding it as that of 
the passage-beds between the Lias and Oolites. The species from 
Midford are marked " M." 
* Quart. Journ. Geol. Soc., Tol. xlvi. pp. 440, 521, &c. ; Journ. Northamptonshire 
Nat. Hist. Soc., vol. v. pp. 77, 7?. 
f Mr. S. S. Buckman now regards them as belonging to different sub-genera. 
J See S. S. Buckman, Quart. Journ. Geol. Soc., vol. xxxvii. p. 608, vol. xlv. 
p. 455, vol. xlvi. p. 520 ; Tate and Blake, Yorkshire Lias, p. 180 ; Fox-Strangwajs, 
Jurassic Rocks of Yorkshire, vol. i. p. 131. 
