46 SILICEOUS SPONGES. 



is represented as one third as large. In the Museum specimen the opening at the 

 summit is nearly one half the diameter, whilst at the basal end it measures only one 

 fourth the thickness of the sponge ; so that at the base, at least, it agrees with 

 Rcemer's description. 



This sponge appears to be a rare form in the Grey Chalk ; there is a fragment of 

 a specimen in the collection inclosed in an Upper Chalk Flint, which also appears to 

 belong to this species. 



Bistrihution. Gray Chalk : near Dover. Upper Chalk : South Coast of England, 

 probably. 



Genus PACHINION, Zittel, 1878. 



Pachinion scriptum, Eoemer, sp. (Plate VII. figs. 1, 1 a, I, c, d.) 



1864. Jerea scripta, F. A. Roemer, Palseont. Bd. 13, p. 34, t. 13. f. 1. 

 1878. Pachinion scriptum, Zitt. Stud. II Ab. p. 66, t. 5. f. 2. 



Sponges simple, cylindrical, or inversely conical, the lower portion of the body 

 gradually tapering to a cylindrical stem ; the summit rounded or depressed conical. 

 Very variable in size — a small specimen measuring 100 mm. in length by 55 mm. in 

 width, whilst a large cylindrical example attains a length of 380 mm. and a width 

 of 68 mm. 



The cloacal tube is approximately cylindrical, and extends nearly the entire length 

 of the sponge; at the summit of a fairly large specimen it is 18 mm. in width. The 

 outer surface is covered by a smooth dermal layer. No distinctive canals are present, 

 the circulation apparently being carried on between the apertures of the spicular 

 fibre. 



The interior of the sponge is formed by a relatively coarse fibrous network ; the 

 fibres are about 0-315 mm. in width. On the surface of weathered specimens and in 

 sections the edges of the fibre have a rude resemblance to hieroglyphic characters. 



The fibre is composed of irregular branching spicules with tubercled surfaces. 

 The dermal layer is formed of much smaller and more branched spicules than those 

 of the fibre. In the specimens from Flamborough only the general characters of the 

 spicules of the fibre can be recognized ; but in examples from Germany the spicules 

 are beautifully preserved. 



This species is very abundant in the Chalk at Flamborough ; the fibre is, for the 

 most part, in a silicified condition, though occasionally the silica is replaced by 

 calcite. A few specimens are also met with in the Chalk of the south of England ; 

 but the siliceous spicular structure of these has been replaced by iron oxide, and is 

 quite as indistinct as in the Flamborough examples. This species can be readily 

 distinguished from the cylindrical specimens of Sci/talia and Plnjmatella in the 

 Flamborough beds by its smooth surface, the coarse characters of the fibre, and the 

 absence of canals. 



