134 SILICEOUS SPONGES. 



> 



1842. Compare Spongus meandrinoides , Leymerie, Mem. Geol. Soc. do France, vol. v. p. 1, 

 1. 1. f. 2. 



Sponges growing in subspherical, ovate, and irregular masses of very various 

 dimensions. A small specimen is 90 mm. in diameter, and a large example measures 

 190 mm. across. The mass is composed of anastomosing folds of wall, from 2*5 to 

 3 mm. in thickness, which in some specimens form wide anastomosing tubes, and in 

 other examples open meandriform folds. There are numerous intermediate examples 

 between the tubular forms and those with open folds, and frequently the two 

 characters are met with in the same specimen. The surface of the wall is perforated 

 on both sides by the apertures of blind canals, from -5 to -75 mm. in width. 



The dermal layer is composed of a minute spicular network with circular or ovate 

 pores ; the spicular mesh of the interior of the wall is irregular in its disposition ; 

 the distance from node to node is -25 mm. ; the lantern character of the nodes is 

 distinctly shown. 



This species differs from the preceding in the disposition of the folds of the wall, 

 and in the closer and less regular character of its interior mesh. Mr. F. G. H. 

 Price* has named this form PlocoscijpMa meandrina, Leymerie, which I presume is 

 the same as the SjJongus meandrinoides of this author. Leymerie's figure, however, 

 appears to me to represent a form differing from Smith's in the thinner walls and 

 closer arranged folds. As the French typical specimen is only a cast in iron pyrites, 

 no other comparison beyond that of outer form and the thickness of the wall-folds is 

 practicable, and in both these features it differs from P. Jahrosa. 



This species is extremely abundant in certain zones of the Upper Green Sand and 

 Chalk Marl. 



Distribution. Upper Green Sand and Chalk Marl : between Folkestone and Dover. 



Plocoscyphia pertusa, Geinitz. 



1843. Tragos pertusum, Geinitz, Die Versteiuerungen von 'Kiesliagswalda, p. 19, t. 6. f. 18. 

 1871. PlocoscjjpMa pertusa, Gein. Palaeont. Bd. 20, p. 26, t. 2. f. 5 a, b, and t. 3. f. 1 a, b. 

 1878. PlocoscijpMa pertusa, Zitt. Handb. der Pal. p. 181, f. 96. 



1883. Plocoscyphia pertusa, Keeping, Fossils of Upware &c. p. 145, t. 8. f . 1 a, 1 b. 



Sponge irregular in form, built up of convoluted walls, about 1-75 mm. in thick- 

 ness, disposed so as to leave wide channels or anastomosing cylindrical tubes, about 

 8 mm. in diameter. The exterior surface is rough and uneven, and is furnished with 

 numerous irregularly distributed circular or elongate canal-apertures. 



The dermal layer is composed of a stout spicular mesh ; the spicular arms are 

 flattened and spinous, and the nodes solid ; the interspaces of the mesh are mostly 

 circular. The interior spicular mesh has subquadrate interspaces. The nodes are 

 lantern-like ; the distance from node to node is -25 mm. 



* Quart. Journ. Geol. Soc. 1877, vol. sxsiii. p. 442. 



