PLATTCHONIA.— SELISCOTHON. 31 



Platychonia, sp. 

 Distribution. Upper Jura: Wurgau, Franconia (Zittel's coll.). 



Genus BOLIDIUM, ZiU. 1878. 



BoLiDiUM PALMATUM, Roemer, sp. 

 1864. Amorphospongia palmata, Roemerj Palseont. Bd. 13, p. 55, t. 19. f. 8. 



Microscopic slide with spicules of the species. 

 Distribution. Upper Chalk : Sudmerberg ; Goslar. 



Genus CHONELLA, Zitt. 1878. 



Chonella tenuis, Eoemer, sp. 



1864. Cupulospongia tenuis, Roemer, Palffiont. Bd. 13, p. 51, t. 17. f. 7. 

 1878. Chonella tenuis, Zitt. Stud. II Ab. p. 52, t. 3. f. 6, 7. 



Distribution. Upper Chalk : Sudmerberg, Biewende ; Brunswick. 



Chonella auriforms, Roemer, sp. 



1840. Achilleum. auriformis, F. A. Roemer, Nordd. Kreide. p. 2, t. 1. f. 3. 

 1878. Chonella auriformis, Zitt. Stud. II Ab. p. 52. 



Distribution. Neocomian : Berklingen, Brunswick. 



Genus SELISCOTHON, Zitt. 1878. 



Seliscothon planus, Phillips, sp. (Plate II. figs. 2-4.) 



1835. Spongia plana, PhilL Geol. of Yorkshire, p. 177, t. 1. f. 1. 



1835. Spongia capitata, Pbill. ib. p. 177, t. 1. f. 2. 



1878. Seliscothon planus et capitatus, Zitt. Stud. II Ab, p. 54. 



Sponge similar in form to an expanded mushroom. The body or upper portion is 

 plate-like, circular in outline, usually with a slight central depression immediately 

 above the stem. The upper surface sometimes shelves gradually from the margin to 

 the centre ; more frequently, however, it is nearly flat or even slightly convex. The 

 margins are usually oblique, and form nearly a right angle with the under surface. 

 The lower surface is flat or slightly concave. The stem is inversely conical in form, 

 and gradually tapers to a blunted extremity. There are no indications of its attach- 

 ment to a foreign body. It varies in length in different examples ; an average 

 specimen measures 55 mm. Small examples of the species are not more than 

 27 mm. in width, whilst large forms reach up to 190 mm. The thickness of the 

 body-plate varies between 10 and 14 mm. 



