POEIFERA (sponges). 



41 



7. 



(Fig. 16 c), the large goblet forms of Pachyi)oterion, and Gallery X. 

 the cylindrical and branching examples of Doryderma 

 (Fig. 16 from near Warminster. 



Next follow the sponges from the Chalk, which, though Table-eases 

 perhaps more numerous, are not so well preserved. Those ■'■•'■» 

 from the Upper Chalk of the South of England have had Wall-case 

 their skeletons almost entirely replaced by iron-peroxide; 

 very frequently also they are now enclosed in nodules of 

 solid flint, in part retaining their forms, but their interior 

 structures are merged in the flinty matrix, and only show 

 the course of the larger excurrent canals and the cloacal 

 cavity. In the sponges from the Upper Chalk of Flam- 

 borough, Yorkshire, the form is usually preserved, and also 

 the main features of the canal system, but the spicular 

 structure is now scarcely recognisable. Calcispongiae are 

 represented by Elasmostoma and Pharetrospongia, but are 

 not numerous except in the case of the little globular 

 Porosphaera, which has recently been placed here by 

 Dr. Gr. J. Hinde. The Hexactinellida, all belong to the 

 Sub- Class Dictyonina, and among them the best known is 

 the funnel-shaped Ventriculites with its folded walls (Fig. 10), 

 but one should notice also the wide flange-like walls 

 of Guettarclia and the mushroom-shaped Coeloptyclmtm. 

 Among Demospongiae, the Choristid 

 Tetractinellida are represented by 

 Pachastrella and Stelletta and numer- 

 ous detached spicules of Geodia (?). 

 Lithistida are numerous, and here 

 may be observed the large size and 

 the projecting canal-openings in 

 Stichophyma and Verrumlina from 

 the Flamborough Chalk (Fig. 17). 

 Monactinellida include the borings of 

 Cliona in Molluscan shells. These 

 last are the only representatives of 

 sponges in the British Tertiary series. 



Passing now to the fossil sponges from foreign 

 localities, exhibited in the Wall-cases, one sees among 

 the Ordovician specimens the large branching Lyssacine 

 Hexactinellid, Brachiospongia digitata from Kentucky. 



The Silurian of North America has furnished the 

 Monactinellid Climacospongia and numerous examples of 

 the Lithistid genera Astylospongia, Palaeomanon, and Hindia. 

 The Lithistid Aulocopmm is from G-otland in the Baltic. 



'}'' 



Fig. 11 —Verruculina 

 Beussi. Senonian 

 Chalk of Flam- 

 borough Head, i 

 nat. size. 



Table-ease 

 11. 



Wall-ease 

 8c. 



Wall-ease 

 8b. 



