DORSET-SOMERSET DISTRICT. 33 
tation in assigning all specimens from P, of the Burton Bradstock Cliff to the 
lower division of that zone—which throughout this neighbourhood is richer 
than all the rest put together. Am. Martinsii is also abundant in this bed, and 
other Ammonites which are characteristic of this horizon. 
According to my experience Am. Martinsii is a pretty faithful guide to the 
geological horizon of a particular bed when it occurs in any quantity. The posi- 
tion of this abundant development is low down in the Parkinsoni-zone, or in some 
cases we might say high up in the Humphriesianus-zone. If we accept for the 
moment the level of maximum development of Am. Martinsii for our datum line we 
shall find that here, at Burton Bradstock, Am. Parkinsoni descends rather lower 
than the former does, for instance, at Oborne, as we shall perceive subsequently. 
Hence we must bring the Parkinsoni-zone a little lower down here, since, not 
only does the section Cosmoceras exceed all other sections of the unwieldy genus 
Ammonites in P,, but the real Am. Parkinsoni is there in abundance quite to the 
base of the bed, but only of moderate dimensions. I have been somewhat precise 
in respect of this bed, not only because it is ‘‘the fossil-bed” of Burton Brad- 
stock and the neighbourhood, but because, in the excellent section contributed by 
Mr. Etheridge to Damon’s ‘ Geology of Weymouth,’ it is evident that this bed is 
regarded as being in the Huwmphriesianus-zone. This misconception is probably 
due to an exaggerated notion of the importance of the Hwmphriesianus-zone in 
South Dorset, and may have its origin in the opinion expressed by the late Dr. 
Wright as to the position of the “ fossil-bed” at Bradford Abbas. 
Although there is considerable variety in the mineral composition of P,, yet the 
matrix of a majority of the Gasteropoda is a pale grey marly limestone, which is 
very characteristic of this bed wherever it occurs between Burton Bradstock and 
Bridport Harbour. According to the degree to which this bed is ironshot, browner 
tints, more characteristic of the Dorset Inferior Oolite, are seen to prevail. On 
the whole, however, the bed as here developed seems to work rather more kindly 
than is often the case, and as the fossils are in good spathic condition, it has often 
been possible to develop them with a considerable degree of accuracy. My collector, 
Mr. Bloomfield, has been very successful in this respect, and specimens of Spinigera 
recurva, sp. n., with most of the spines attached, and other good fossils have rewarded 
his efforts. As we shall perceive in the sequel the Gasteropod Fauna of this bed 
is very distinct, and may be traced for some distance in South Dorset, nor is it 
difficult to see what are its approximate equivalents in other parts of No. 1 District. 
Moreover, its Gasteropoda are clearly those of the Bajocian (understanding this 
term to include the Humphriesianus- and Parkinsoni-zones only). They are fairly 
different from the much lower Bajocian beds at Oborne, and present a striking 
contrast to those of the infra-Bajocian “ fossil-bed ” of Bradford Abbas, which, as 
we have already seen, belongs to Deslongchamps’ “ Marnes Infra-odlithiques.” I 
5 
