HOLODICTYON.— PACHTPOTEEION. 51 



those of Doryderma in the character and disposition of the spicular mesh, but, unlike 

 that genus, appear to be destitute both of vertical and horizontal canals, so that the 

 circulation is wholly carried on between the interspaces of the mesh. 



HoLODicTYON CAPiTATDM, IHiide, n. sp. (Plate VII. figs. 3, 3 a, 3 b.) 



The examples of this species vary from 100 to 150 mm. in length, and from 30 to 

 85 mm. in width at the summit. The cloacal aperture, if it may be so termed, is 

 from 6 to 9 mm. in width at the summit ; in one example, which has been cut in two, 

 its depth is 20 mm. The spicules are robust, and measure about '2 mm. in thickness. 

 No dermal layer has been detected. The spicules of the stem appear to be similar 

 to those of the body of the sponge. The interstitial apertures are irregular in form, 

 and from '5 to '75 mm. wide. 



Distribution. Upper Green Sand : Warminster, Wiltshire. 



Genus PACHYPOTERION, Hinde, n. g. 



Sponges simple, massive, with cup- or goblet-shaped bodies, frequently of great 

 thickness, supported on elongated cylindrical or compressed stems. The body of the 

 sponge is traversed by a double series of canals — one, rising from the basal portion, is 

 either vertical or follows the contour of the cup and opens into it; the other, 

 and smaller, series commences on the exterior of the cup, and extends downwards in 

 an arched direction towards the centime of the sponge. The skeletal spicules of the 

 body of the sponge are smooth, curved, branching bodies of very irregular form ; the 

 extremities of the arms are either blunted or slightly expanded. They form a compact 

 network both by the intertwining of the arms and by the attachment of the expanded 

 ends to the surfaces of adjoining spicules. The spicules of the stem are elongated, 

 thread-like, bifurcated at their ends, and loosely connected together. No dermal 

 spicules have been preserved. 



From Doryderma this genus is distinguished mainly by its cup-shaped mode of 

 growth. It differs from Heterostinia, Zittel, in the apparent absence of those minute 

 spicular bodies in which, according to Zittel, the larger spicules of this latter genus 

 are imbedded, and which form the principal mass of the skeleton. It is possible 

 that these smaller spicular elements may have been originally present in the examples 

 of Pachypoterion ; but, considering the state of preservation of these sponges, I think 

 that traces of them would have remained. 



The examples of this genus at present known are limited to the Upper Green 

 Sand. On account of their resemblance in external form and in the canal-apertures 

 of the inner and outer surfaces to Chenendopora, these sponges have hitherto been 

 placed under that genus, and only by an examination of the spicular structure can 

 the differences be detected. 



h2 



