110 SILICEOUS SPONGES. 



1848. Ventriculites muricatus, T. Smith, Ann. & Mag. Nat. Hist. vol. i. p. 210, t. 13. f. 1. 

 1878. Ventriculites dilatatus, Queust. Petref. Bd. 5, p. 460, t. 137. f. 6. 



Sponge usually open funnel-shaped, gradually tapering to a slender stem with 

 divergent rootlets. In some instances the sponge forms flattened expansions with a 

 circular outline similar to V. radiatus. Height and width very variable ; a large 

 specimen is 170 mm. in diameter. 



The outer or under surface of the sponge-wall is reticulate, with circular or ovate 

 apertures, from "75 to 1-5 mm. in width ; the wall-fibres bounding the apertures are 

 about the same width as the interspaces. The upper or inner surface has numerous 

 apertures of a similar form to those of the lower, and disposed either irregularly or 

 in quincunx. 



The total thickness of the wall is about 3 mm.; but as all the specimens are in 

 chalk, and therefore compressed, it is impossible to determine the original thickness 

 with accuracy. No spicular structure is shown in any of the specimens. 



This species appears to be nearly as abundant as the preceding. There is a great 

 amount of variation in different specimens, but I cannot find any characters which 

 would allow of a separation into distinct species. The V. muricatus, T. Smith, 

 appears to me, from a comparision of the original example in the Museum Collec- 

 tion, to belong to this species. 



Distribution. Upper Chalk : South of England, Heytesbury ; near Warminster, 

 Wiltshire {coll. T. Smith). 



Ventriculites convolutus, Hinde, n. sp. (Plate XXV. figs. 5, 5 a.) 



Sponges cup- or vase-shaped, with expanded margins and folded walls. No stem 

 preserved, and the species appears to have been sessile. The wall of a specimen 

 preserved in chalk is 1-6 ram. in thickness. A fairly large specimen is 60 mm. in 

 height, and 120 mm. in breadth at the summit. 



The outer surface is formed of flattened, bifurcating ridges about 1-5 mm. in width, 

 which run nearly parallel with each other from the base to the margins, and inter- 

 osculate so as to leave narrow, elongated interspaces. The interior of the wall is 

 not exposed. 



The spicular mesh, judging by the hollow casts in the Chalk, is irregular; the 

 distance between the spicular nodes is about 3 mm. This species may be distin- 

 guished by its folded thin walls and the disposition of the flattened ridges of the 

 outer surface. 



In outer form and the disposition of the ridges of the outer surface of the wall, 

 this species resembles Crihrospongia subreticulata, Geinitz*; but the surface-apertures 



* Palceontographica, Bd. 20, p. 23, t. 2. f. 2-4. 



