HEADON HILL AND COLWELL BAT IN THE ISLE OF WIGHT. 93 



Cliff, and again in Colwell Bay, both physically and as to its fossils ; 

 when we come to the portion of the series richest in Cytherea incras- 

 sata known as the " Yenns-bed " proper, we find identically the 

 same fossils as in Colwell Bay ; we cannot detect any difference, so 

 far as our researches go. 



The uppermost bed is dark greenish clay, varying from 9 inches 

 to 2 feet in thickness. When at its thickest the sands above are 

 proportionally thinner. It is extraordinarily rich in fossils for the 

 depth of one foot, chiefly Neritina concava, Cerithium (Vicarya) 

 concavum, C. ventricosum, Corbicula obovata, Marginella vittata, 

 Melania muricata, Melanopsis fusiformis. "We may call it the C. ven- 

 tricosum bed ; for this fossil seems almost confined to this horizon, 

 while the Vicarya concava is found here all through the Middle 

 Headon, though especially plentiful in this bed ; its underside is 

 occupied by an impure lignite band, with freshwater shells (Limncea 

 and Planorbis), and rootlets, lying on a thin crumbly buff Limnsea- 

 limestone. Both together are 4 to 6 inches. Below is verdigris- 

 green clay, 6 inches ; next a buff freshwater limestone with Limncea, 

 1 foot 8 inches to 2 feet, the shells frequently blackish in colour. 

 These beds betoken certainly a recurrence of freshwater conditions 

 after the brackish bed above. Next follow truly marine beds — first 

 stiff green and grey clay, about 9 feet, with a conchoidal fracture 

 when dry. The abundance of oysters, 0. velata (Wood), is the chief 

 fossil feature ; these and the other fossils occur mostly towards the 

 base. Such fossils are Fusus (Pisctnia) labiatus, Nerita ajoerta, Me- 

 lania fasciata, M. muricata, Cerithium variabile, C. pseudocinctum, 

 Mytilus affinis, Corbicula obovata. 



The fossils cited are merely the most common, such as may be 

 found in a few minutes' search ; but these being the most charac- 

 teristic, seem to us precisely those required for the identification 

 and correlation of beds. 



Comparing the beds noticed so far with those at Colwell Bay, 

 we observe practically identity of fossils ; this last, which we may 

 call the oyster-bed, is identical with the oyster-bed of Colwell Bay ; 

 in both localities C. ventricosum occurs above this bed, and there 

 only, so far as we know. 



Next below follow alternations of grey and ochry silts, 3 feet, in 

 which we observed no fossils. Below are ] 1 feet of beds, brown 

 sandy clay above becoming green clayey sands below, and then grey 

 sandy clays : this is the " Venus-bed " of collectors ; Cytherea incras- 

 sata occurs near the top and is scattered throughout the whole bed, 

 but is most abundant for the space of one foot. The fossils obtained 

 from the Venus-bed in a few minutes' search were Cytherea incras- 

 sata, Mya angustata, Mactra fastigiata, Psammobia ruclis, P. azstu- 

 arina (Ed. MS.), Nucula headonensis, iV. nudata, Trigonoccelia clel- 

 toidea, Corbicula obovata, Vicarya concava, Ancillaria buccinoicles, 

 Fusus labiatus, Cancellaria elongata, Natica Studeri, N. labellata, 

 Voluta S]pinosa, Melanopsis fusiformis, Nematura parvula, Limnwa 

 longiscata, Planorbis obtusus, crab-claw {Callianassa), coprolite. 



