A. S. Lucas— The Headon Beds of the Isle of Wight. 103 



Blake, although it is decisive,^ as it is impossible, until long 

 acquaintance with the ground makes the exact position of the 

 smaller zones on all parts of the coast familiar, to make anything 

 like an exhaustive collection. We cannot adduce lists of the com- 

 pleteness of those of Messrs. Keeping and Tawney,^ although we 

 spent much time and pains to find as many forms as might be, but 

 in some particulars our evidence is in positive confirmation of their 

 observations. There are three lines of argument adopted hj the 

 authors at issue in correlating or separating these two brackish- 

 marine series. First, there is a comparison of the life-zones of the 

 two series. It is claimed by the defenders of the Survey that in 

 both are five beds, characterized in descending order by certain 

 lithological characters and respective predominance of Cerithium 

 ventricosum, Sow. ; Ostrcea velata, Wood ; Cytherea incrassata, 

 Sow. ; TrigonocoBlia deltoidea, Lam. ; and Neritina concava, Sow. 

 We have noted in our own section the brackish forms above and 

 below, and the distinct oyster-bed about the middle. Dr. Wright 

 also notices the Neritina bed. Secondly, the aggregate of forms 

 found at Col well Bay is compared with the aggregate of forms at 

 Headon Hill. Messrs. Keeping and Tawney's lists of the commoner 

 fossils give 55 forms from the former, and 54 from the latter, and 51 

 occur in both. We found, in addition, Cerithium duplex (Sow.) in 

 both localities, which gives out of 55 commoner species 52 occur- 

 ring in both series. ]n the complete fauna as known Dec. 1880, 

 Messrs. Keeping and Tawney, after a careful re-examination of the 

 Edwards Collection, reckon 74 per cent, of the Colwell Bay marine 

 species as having been found at Headon Hill. This may point to 

 more purely marine conditions in the N., agreeing with the presence 

 of the coral fauna at Brockenhurst, as already admitted by Dr. 

 Duncan,^ who quotes other evidence as to the identity of fossils 

 at Colwell Bay and Headon Hill. Thirdly, the different distribu- 

 tion of the two Cerithia (0. ventricosum and C. concavum) is made a 

 crucial point. We found both species in the Colwell Bay beds, as 

 did Messrs. Keeping and Tawney. Our specimens of C. concavum 

 occurred a little below the oyster-bed, between Colwell Chine and 

 Warden Point, a position in which the shell was noted by us N. of 

 Alum Bay Chine. They found them in " the richest portion of the 

 Yenus-bed." Its absence from Colwell Bay is therefore disproved. 



The result of our observations, then, is to convince us that on 

 many crucial points the older views are correct, and that the 

 brackish-marine of Colwell and Totland Bays cannot be continuous 

 with supposed marine beds above the Great Limestone, nor with the 

 Great Limestone itself, nor with the fresh-water sands (containing 

 Unio Solandri, Faludina, etc.) below the Limestone, but corresponds 

 in position, lithology, and fossil contents, with the brackish-marine 

 of Headon Hill, and that it is restricted to this correspondence 

 moreover by the parallel correspondences of the beds below and 

 above it. 



1 op. cif. p. 156, note. ^ g^, ^jY. p. i05. 



3 jjuncan, Mon. Pal. Soc. pt. 1, p. 40. 



