CALLODICTTON.— POEOCHONIA. 143 



base. The stem has not been preserved. An imperfect specimen measures 110 mm. 

 in length, and 40 mm. in width at the summit. 



The wall of the sponge, about 2 mm. in thickness, is composed of an extremely 

 regular spicular mesh with oblong interspaces ; the nodes are octahedral, and about 

 •6 mm. apart in a vertical direction and -45 mm. transversely. The outer surface is 

 formed of a very delicate open network, with circular or ovate apertures, -5 mm. in 

 width, disposed in vertical rows. These apertures open directly into the interspaces 

 of the mesh, and do not appear to be connected with distinct canals. The interior, 

 or cloacal surface of the wall, does not appear to possess a modified spicular mesh. 



The examples of this species are preserved in chalk, and the siliceous spicular 

 mesh has been replaced by iron peroxide, so that it is impossible to determine 

 whether the rays were smooth or spinous. 



Cylindrospongia membranacea, Quenst.*, non Roemer, appears to belong to the 

 genus Callodictyon, and resembles the present form in the disposition of the apertures 

 of the outer surface ; its walls, however, judging from the figure, are not more than 

 one third the thickness of those of C. angustatum. 



Distribution. Upper Chalk : South of England {coll. Bowerbank). 



Genus POROCHONIA, Hinde, n. g. 



Sponges funnel-shaped, with thin walls. The outer surface of the wall is formed 

 by a very delicate membrane, beneath which is a reticulate layer with circular 

 apertures. Below this layer is the regular spicular mesh of the wall ; tlie nodes of 

 the six-rayed spicules are octahedral. The inner or cloacal surface of the wall is 

 formed by a reticulate layer similar to that overlying the spicular mesh. Definite 

 canals do not seem to be present, the circulation being carried on apparently through 

 the regular interspaces of the mesh. 



I have based this genus on the characters of Ventriculites simj)lex, T. Smith, the 

 walls of which differ from those of typical forms of Ventriculites in being smooth 

 and not arranged in vertical folds. From Callodictyon this genus is characterized by 

 possessing a delicate surface-membrane (the polyp-skin of T. Smith) in addition to 

 the usual dermal layer (the underskin of T. Smith). The exterior membrane is only 

 present in some of the best-preserved specimens ; and owing to the replacement of 

 its original spicular structure by iron peroxide, the minute characters cannot be 

 determined. 



PoROCHONiA SIMPLEX, T. Smith, sp. (Plate XXX. figs. 5, 5 «, 5 b.) 



1848. Ventriculites simplex, T. Smithy Ann. & Mag. Nat. Hist. 2nd ser. vol. i. p. 204, and 

 1st ser. vol. xx. t. 8. f. 1. 



* Petref. Bd. 5, p. 46S, t. 137. f. 13. 



