VENTKICULITES. 109 



Sponge consisting of a circular, flattened, or slightly concave, or occasionally 

 convex expansion, supported on a small funnel-shaped or inverted conical base, from 

 the extremity of which divergent root-like processes extend, which served to anchor 

 the sponge in the chalky mud. 



The conical basal portion is from 20 to 30 mm. in height ; the summit expansion 

 attains, in fairly large examples, a diameter of 220 mm. 



The under surface of the sponge exliibits a series of straight or slightly curved, 

 occasionally bifurcating ndges, which radiate from the centre to the margins. These 

 ridges are formed by the iufolding of the thin wall-plaits of the sponge ; they are 

 connected together at intervals by transverse extensions. They are from 1"5 mm. to 

 2'5 mm. in tliickness, and the intervening furrows are about 1 mm. in width. 



The upper surface of the sponge has a smooth aspect ; it is composed of a reticu- 

 lated membrane with numerous circular or elliptical canal- apertures, from 1'5 to 

 3 mm. in width, disposed either irregularly or in quincunx. The total thickness of 

 the sponge-wall vai-ies according to the amount of compression it has experienced ; 

 in some examples in the Chalk it is only 2 to 3 mm., whilst in a flint-preserved 

 example it reaches a thickness of 6-5 mm. 



In only a single specimen out of a large number of examples of this species have 

 I been enabled to discover traces of the spicular structure. The spicular mesh in 

 this specimen appears to be extremely regular ; the interspaces are nearly quadrate, 

 and the distance between the spicular nodes is about '4 mm. 



This species is probably the most abundant of any of the Ventriculites in the 

 English Chalk. It occurs either as mere impressions or as moulds filled with soft 

 ochraceous material in the chalk, or else as casts in flint. Very frequently the 

 basal or conical portion of a specimen is preserved in flint, whilst the expanded 

 portion remains as a mere impression in the soft chalk. Its appearance varies 

 greatly according to its mode of preservation, and the surface which is exposed 

 to view. 



1 have taken as the type of the species the specimens figured by Mantell on 

 tab. 14, 'Fossils of the South Downs.' The figures, though roughly drawn, exhibit 

 very fairly both the upper and under surfaces of the sponge. The examples figured 

 by Mantell on tab. 13 belong, with one doubtful exception, to other species of 

 Ventriculites, and were so regarded by Mantell himself in the ' Medals of Creation,' 

 p. 245, f. 82. 



Distribution. Upper Chalk : Norfolk ; Surrey ; Sussex ; near Warminster, Wilt- 

 shire {coll. T. Smith and Mantell). 



Vkntriculites impressus, Touhn. Smith. 



1848. Ventriculites impressus, T. Smith, Ann. & Mag. Nat. Hist. 2nd ser. vol. i. p. 205; 

 and 1st ser. vol. xx. t. 8. f. 2, 3. 



