. Bulletin of the British Museum (Natural History), Geology. CRETACEOUS ECHINODERMS FROM WILMINGTON 29 number of specimens as %. 25 50 75 91.*' •. ll Tiaromma Polydiadema Tetragramma Allomma Fig. 9 Quantitative analysis of the distribution of the four species of pseudodiadematid present at Wilmington. Units a to h are as in Fig. 5 and are drawn in proportion. The changes in the abundance and species composition of the echinoid fauna collected from this stratigraphical succession are summarized in Figs 7-9. The composition of the echinoid fauna changes markedly at two levels, one at around
. Bulletin of the British Museum (Natural History), Geology. CRETACEOUS ECHINODERMS FROM WILMINGTON 29 number of specimens as %. 25 50 75 91.*' •. ll Tiaromma Polydiadema Tetragramma Allomma Fig. 9 Quantitative analysis of the distribution of the four species of pseudodiadematid present at Wilmington. Units a to h are as in Fig. 5 and are drawn in proportion. The changes in the abundance and species composition of the echinoid fauna collected from this stratigraphical succession are summarized in Figs 7-9. The composition of the echinoid fauna changes markedly at two levels, one at around 900-940 cm below datum level and the other at zero datum level. The second of these coincides with a major break in sedimentation and the overlying muds were inhabited by a fauna quite different from that found in the muddy sands of the Cenomanian. Throughout the Wilmington Sands and Grizzle, however, species composition does not really change, although the relative abundance of each species is highly variable. The two species that do make an apperance towards the top of the Grizzle (Conulus castanea and Holaster bischoffi) appear suddenly in the succession without obvious antecedents and almost certainly migrated in when conditions at Wilmington became favourable. Indeed, one of the species, Conulus castanea, is known from the Albian of France and its sudden appearance in this section towards the top of the Grizzle can only be through migration. The variation in relative abundance of the different species within this part of the succession is therefore best seen as resulting from changing environmental conditions. At the base of the section there is a moderately diverse fauna of sediment-feeding irregular echinoids and an unusually low proportion of epifaunal forms, particularly regular echinoids (though this may be partially the result of rather small sample size). This fauna is replaced in the lower part of the Wilmington Sands by one dominated by the generalist feeder Dis
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