136 . SILICEOUS SPONGES. 



Plocoscyphia convoluta, Toulm. Smith, sp. 



1848. Brachiolites convolutus, T. Smith, pars, Ann. & Mag. Nat. Hist. vol. i. p. 355, t. 15. 

 f. 5, uon fig. N. p. 357. 



Sponge growing in small masses of irregular form, composed of delicate convoluted 

 walls about 1 mm. in thickness. Traces of a minutely perforated dermal layer are 

 present in one example ; the interior of the wall is built of an irregular slender 

 spicular mesh ; the distance between the nodes is -25 mm. The spicules have been 

 replaced by iron peroxide, and the character of the nodes cannot be determined. 



Toulmin Smith states that "the sti'ucture of the simple membrane (or wall) in 

 this species seems coarse — that is, the squares (of the mesh) are larger than in any 

 other species of the Ventriculitidse." Such, however, is not the case in the specimen 

 which he has figured, and which I have taken as the type of the species. The inter- 

 spaces of the mesh in this example are relatively small. In the original specimen 

 there are no traces of the minute stem and root represented in the figure given by 

 Smith. 



IHstribntion. Upper Chalk : South of England [coll. T. Smith). 



Plocoscyphia flexuosa, Mantell, sp. (Plate XXIX. fig. 4.) 



1822. Choanites flexuosus, Mant. Foss. of the South Downs, p. 179, t. 15. f. 1. 



1848. Brachiolites convolutus, T. Smith, pars, Ann. & Mag. Nat. Hist. vol. i. p. 357, fig. N. 



1878. Gyrispongia labijrinttiica, Quenst. Petref. Bd. 5, p. 485, t. 138. f. 12, 13. 



Sponges inverted conical, or irregular in form, in some instances supported on a 

 cylindrical stem with divergent rootlets at its termination. The body of the sponge 

 is composed of convolute anastomosing walls from 1-25 to 1-7 mm. in thickness. The 

 surface of the walls has numerous circular canal-apertures, '6 mm. in width. The 

 spicular mesh of the interior is somewhat irregular in disposition ; the distance 

 between the octahedral nodes, or the diameter of the interspaces of the mesh, varies 

 between 'ob and '4 mm. 



The type of this species, now in the Museum, is the lower portion of a specimen 

 preserved in flint, showing a section of the convolute walls and traces of the spicular 

 mesh. Numerous other examples, both in flint and in chalk, exhibit walls of the 

 same thickness, and a spicular mesh with interspaces of the same size as those of the 

 type. In none of the specimens has the outer form been completely preserved ; so 

 that the only characters available for specific detern)ination are the thickness of the 

 walls and the dimensions of the mesh interspaces. The sponges figured by Quenstedt 

 under the name of Gyrispongia labijrinthica appear to correspond with the present 

 species ; but I am unable to determine whether the Plocoscyphia labyrinthica of 

 Reuss* and F. Roemerf belong to it. 



Distribution. Upper Chalk : South of England {coll. Mantell, T. Smith). 



* Bohm. Kreide, II Ab. p. 77, t. 18. £. 10. t Geol. Oberschles. p. 309, t. 33. f. 7. 



