40 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



G-allery X. preserved Calcisponges, of which a fine series is shown. The 

 chert beds of this age in Kent and Surrey are mainly com- 

 posed of detached spicules of Tetractinellida and Lithistida. 

 A few larger fragments of the latter group from Sevenoaks 

 are the only representatives of the complete skeleton. From 



Table-ease the Gault there are only some specimens of Jerm and a fine 

 example of the Hexactinellid genus Gratimlaria in pyrites. 

 The Upper Greensand and the Chalk Marl, on the other 

 hand, contain great numbers, which preserve their original 



Table-eases form and structure. Those from the Greensand of War- 

 13, 14. 



Fig. 16. — Lithistid sponges from the Upper Greensand of Warminster, 

 a, Hallirhoa costata (specimen 48174) ; b, Doryderrna dichotomum 

 (specimen P 1260) ; c, Siphonia tulipa ; a section through the upper 

 part of the sponge, showing the cloaca in the middle near the top, the 

 large excurrent canals which open into it, and the fine incurrent 

 canals leading from the outside to these latter (specimen P 1366). 

 a and b are reduced to ^ nat. size, and c to ^. (After Hinde. See 

 Table-case 13.) 



Wall-ease minster, Wilts, first studied by Miss Etheldred Benett of 

 that place, are shown in great quantity. Amoug the Calci- 

 Tabl^-ease gp^j^g^^g ^qq^ specimens of Fharetrospongia Strahani 

 Table-ease from Cambridge. The folded walls of Plocoscyphia are 

 1^- conspicuous among the Hexactinellida, but the Lithistid 

 Tetractinellida are the most richly represented. Among 

 these last, particular attention may be directed to the 

 peculiar lobate forms of the Lithistid Hallirhoa (Fig. 16 a) 

 with long stems, the perfect Siphoniae from Blackdown 



