PLACONELLA. — DOEYDEEMA. 47 



Distribution. Upper Chalk : Flamborough ; south of England ; Schweichelt, 

 Brunswick (Zittel's coll.). 



Family MEGAMORINA, Zittel. 



Genus PLACONELLA, Hinde, n. g. 



Sponges sessile, growing in flattened, cake-shaped masses. The upper surface 

 uneven with shallow depressions, containing the apertures of seveial prominent 

 canals. The surface between the depressions faintly furrowed, and with openings 

 of numerous canals of diff'erent sizes. The interior permeated irregularly by canals. 



The skeleton consists of smooth, robust, irregularly branching spicules, which 

 closely intertwine together and form a reticulate mesh. 



The spicules and the general structure of the skeleton of this genus are similar to 

 to those of Megalithista, from which it is distinguished by its mode of growth and 

 the absence of a central cloaca. 



PLACONELLA PEKFORATA, Hinde, n. sp. (Plate VII. figs. 2, 2 a, b.) 



The only example of the genus has a circular outline about 68 mm. in diameter, 

 and is 35 mm. in thickness. The upper surface is gently convex, the central portion 

 is elevated ; between this and the margins are three well-marked depressions, and one 

 or two minor ones ; in these depressions are several irregularly disposed canal-apertures 

 about 2 mm. each in width. The general surface is also covered with slight furrows 

 and irregularly disposed canal-apertures, in addition to the openings between the 

 spicular mesh. 



The spicular skeleton is best shown on the surface of the specimen. A detached 

 spicule which I have been able to measure is 1 mm. in length by T5 ram. in width. 

 No distinctive dermal layer has been preserved ; there are, however, in the depressions, 

 fragments of long, straight, cylindrical spicules which may perhaps belong to the 

 sponge. 



The specimen in the collection was labelled Achilleum tuberosum, Goldf.; but this 

 species has a skeleton of Ehizomorine spicules, and belongs to the genus Cnemidiastrum. 



Distribution. Upper Jura : Nattheim, Wiirtemberg. 



Genus DORYDERMA, Zittel, 1878. 



DoRTDERMA DicHOTOMUM, Benett, sp. (Plate VIII. figs. 1, 1 «, 1 b.) 



1831. Polypothecia dichotoma, Benett, Catalogue of the Organic Remains of the Countj- of 



Wilts, pi. 13. 

 1864. Non Pohjjerea dichotoma, F. A. Rosmer, Palseont. Bd. 13, p. 36, t. 14. f. 1 c. 



Sponge dendriform, growing from a simple upright cylindrical stem, which 



