LOWER CHALK— CHARACTERISTIC FOSSILS. 
31 
Echinodermata. —These are present in great variety and abun¬ 
dance, some of the species being specially characteristic of the Lower 
Chalk, though many of them make their first appearance as rare 
fossils in the very highest stratum of the Upper Greensand. The 
following is a list of the principal species : — 
Cidaris Bowerbanki, Forbes 
„ vesiculosa, Brongn. 
Discoidea cylindrica, Lam. (Fig. 15) 
„ subuculus, Klein . 
Echinocyphus difficilis, Gott. 
Galerites castanea, Brongn. 
Hemiaster Morrisi, Forbes. 
Holaster las vis, Deluc. (= carinatus, Ag.) 
„ subglobosus, Leslce (Fig. 14) 
,, trecensis, Leym. 
(Master sphsericus, Schlilt 
Peltastes clathratus, Ag. 
Pseudodiadema ornatum, Goldf. 
„ variolare, Brongn. 
Pentacrinus Agassizi, Hag. - f 
Salenia Clarki, Forbes 
„ Austeni, Forbes 
„ petalifera, Desm. 
Actinozoa.—T he conditions prevalent in the sea of the Lower 
Chalk were unfavourable to the growth of Corals, and the only 
common form is a small flat cup-coral, Micrabacia coronula ; another 
very small and elongate coral, Onchotrochus serpentinus, is also of 
occasional occurrence. 
Spongida.— The remains of siliceous sponges are very abundant 
in some localities, Craticularia Fittoni and Plocoscyphia labrosa 
sometimes forming reef-like layers in the lower part of the Lower 
Chalk. The following are some of the commoner species : — 
Craticularia Fittoni, Mant. 
Heterostinia obliqua, Benett 
Nelumbia tuberosa, Hinde 
Phymatella intumescens, Roem. 
Placotrema cretaceum, Hinde 
Plocoscyphia labrosa, Smith 
Siphonia ficus, Goldf. 
Strephinia convoluta, Hinde 
Stauronema Carter!, Sollas (see Figs. 20 and 21). 
The last-mentioned is specially characteristic of the true Chloritic 
Marl, and does not range far above that bed. 
