The Geology around Oxford. 319 



upper part. The con-ventional division into Portland Sand and (over- 

 lying) Portland Stone appears applicable only to portions of this area, 

 and even then it is better to use the terms Lower and Upper Portland 

 Beds, as both divisions contain beds of sand, and in certain tracts 

 the Lower beds pass into a loamy clay equivalent to the Hartwell 

 Clay." In the explanatory column the Hartwell Clay is shown as 

 part of the Lower Portland Beds, though on the map it is not possible 

 to separate it from the Ivimeridge Clay. 



From the upper beds (sands and limestones) of Shotover the 

 following are amongst the fossils quoted, viz., Pernfliinctes hononiensis,^ 

 de Lor., Cerithium porilandicum, Natica elegans, Pecten lamellosus, and 

 Trigonia gilbosa, all characteristic Upper Portlandian species. From 

 the lower beds the following are amongst the fossils recorded, viz., 

 Perisphinctes pecti7iatus, Phil., Pholadomxja tumida, and Trigonia Pellati. 

 This is rather a scanty list, but the Clavellate TrigonicB are 

 characteristic Lower Portlandian species almost as much as the 

 GlalrfB are characteristic of the Upper Portlandian, where some 

 palaeontologists would lump the several varieties under Trigonia 

 gihhosa, Sow. In the outlier at Brill the boundary drawn at the base 

 of the Portland Beds is in reality at the junction of the Upper 

 Portlandian with the Hartwell Clay (Lower Portlandian), which 

 merges downwards into the Kimeridge Clay. The base of the Upper 

 Portlandian here is the highly fossiliferous glauconitic bed, in which 

 J. F. Blake in 1893 found a series of what he regarded as Lower Port- 

 landian fossils, including Ammonites hiplex,'^ though some of the fossils 

 quoted are likewise characteristic of the higher beds. 



It may be said with a considerable degree of truth that, when 

 correlation is attempted, the faunal history of the Kimeridge- 

 Portland period does not tally with its lithology, although, if we 

 confined our attention solely to the area included in the Oxford map, 



1 This form is sometimes taken for Am. glganteus, Sow., which, fossil Blake 

 showed to be restricted to the. highest Portlandian beds of Portland Isle. AVas Blake 

 justified in altering bononiensis into bolo/iiensis'i 



- It is interesting to observe that fifteen years ago so good a palaeontologist as the 

 late Professor Blake continued to call this characteristic Lower Portlandian fossil 

 by the name of biplex. An equally distinguished paleontologist, de Loriol, gave 

 a very good figure of it, but unfortunately also referred the Boulogne fossil to 

 A. biplex, Sowerby. The Russians Nikitin and Pavlov, when they were over here 

 twenty years ago, had pointed out that our Portlandian fossil was identical with 

 A. Pallasiamis, d'Orb. (in Murchison's " Russia"), which Pavlov then classed under 

 Ferisphinctes. Blake (Q.J. G.S. for 1902, p. 300) referred to a Peri-iphincles (like 

 Fallasi, usually called Am. biplex). Subsequently Miss Healey (Q.J.G.S. for 

 1904) threAv much light on Sowerbian nomenclature, and gave two excellent figures 

 of Olcostephanus Pallasianus, d'Orb., in which we recognize a striking likeness to 

 our old Kimeridge-Portland friend — a very biplex ammonite, but not Sowerby's species. 



It must not be supposed, however, that the stratigrapliist in search of a name for 

 a zonal fossil is quite out of his difficulties. If palaeontologists had been content 

 not to subdivide the ammonites, there would have been no further cause for doubt. 

 But are Ave quite certain that the "true biplex,''^ i.e. Am. Pallasianus, belongs to 

 the genus Olcostephanus ? Many of the specimens of " biplex " in oiu: rocks, owing to 

 their evolute character and the presence of strictures, more resemble Perisphinctes. 

 Moreover, we knoAv that Neumayr, who is responsible for Olcostephanus, essentially 

 a Cretaceous group, admitted that there were intermediate forms. It is significant 

 also that he did not include Am. Pallasianus, d'Orb., amongst the Olcostephani. 



