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 Hamphriesianus-zoue. 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 Parlcinsoni-zone a little lower down here, since, not 

 only does the section Cosmoceras exceed all other sections of the unwieldy genus 

 Ammonites in P 19 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 Hum/phriesianus-zone. This misconception is probably 

 due to an exaggerated notion of the importance of the Humphriesianus -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 1} 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-oulithiques." I 

 5 



