PHTMAPLECTIA. 87 



and in the absence of the dermal layer, the main point of distinction between these 

 forms consists in the much larger size of the interior spicules of the present species ; 

 when the dermal layer is present, the difference between the scaly spicules and the 

 trifids of P. irregularis at once distinguishes these forms from each other. 

 Distribution. Upper Chalk : Wiltshire. 



Genus PHYMAPLECTIA, Hinde, n. g. 



Sponges with plate-like walls, palmate or digitate, or, by the coalescing of the 

 margins of the convolute walls, becoming cup- or funnel-shaped. Canal-system but 

 slightly developed, the circulation apparently being carried on through the openings 

 of the spicular mesh. The interior skeleton is composed of irregular tuberculated 

 spicules of the same type as in PlinthoseUa and Spongodiscus, which are connected 

 together by the interlocking of minute twig-like extensions at the ends of the arms, 

 or by the apposition of adjoining spicules. A dermal layer is usually present, and is 

 chiefly composed of trifid spicules with minute, horizontally expanded, and slightly 

 bifurcated head-rays. 



The character of the dermal layer distinguishes this genus from PlinthoseUa ; in 

 form and mode of growth it differs from Spongodiscus, Zitt. At present no dermal 

 layer has been discovered in sponges of this latter genus, although there is hardly a 

 doubt that one was present originally. 



The forms of the genus at present known are all from the Upper Chalk: 



Phtmaplectia irregularis, Hinde, n. sp. (Plate XXI. figs. 1, 1 a.) 

 Sponges growing in upright fan-like, semipalmate, or digitiform expansions, or 

 occasionally, by the involution of the wall-plate and the coalescence of the margins, 

 becoming cup-shaped. The margins of the wall are rounded ; at the base there are 

 traces of root-like extensions. The specimens are from 100 to 120 mm. in height ; 

 the walls are between 5 and 7 mm. in thickness. 



No canal-system is apparent, but the condition of the specimens is such that it is 

 impossible to determine whether canals were originally present or not. The interior 

 spicular mesh is composed of irregular warty spicules, the arms of which are from 

 •2 to '3 mm. in length. A smooth dermal layer apparently covered the entire outer 

 surface of the sponge-wall, though only portions of it now remain. It is made up of 

 trifid spicules with horizontal heads ; the rays are slightly compressed and regularly 

 bifurcated. The heads of the lai'ger spicules are between '4 and '5 mm. in expansion. 

 The character of the shafts is not shown. 



All the examples of this species are preserved in the interior of flints. In outer 

 form they somewhat resemble the figures of PoUjpothecia palmata and P.fissa, 

 Benett* ; but in the absence of all knowledge of the minute structure of these 

 * Cat. Org. Rem. County of Wilts, pi. 11. f. 2, and pi. 12. 



