132 SILICEOUS SPONGES. 



Museum at Cambridge. The specimen is preserved in a matrix of micaceous 

 sandstone. 



Distribution. Silurian. Upper Ludlow : Brigsteer, Kendal, Westmoreland. 



Genus PLECTOUERMA, Hinde, n. g. 



Outer form of sponge unknown ; the only structures preserved are fragments of a 

 thin spicular membrane, consisting of cruciform and five-rayed spicules of relatively 

 large size, which are grouped together into continuous rows by the apposition and 

 overlapping of several of their vertical axes, whilst the rays of their lateral axes 

 extend singly on either side of the vertical rows and overlap those from the adjoining 

 rows, thus forming an irregular framework. Smaller spicules are present between 

 the interspaces of the larger forms, but they do not appear to be arranged in definite 

 order. 



This genus is closely related to Protospongia, Salter, and Bictyoiihyton, Hall, but 

 it may be readily distinguished from either of these genera by the less regular 

 character of the spicular framework and the absence of definite larger and smaller 

 squares. From Protosjwngia it is further distinguished by the disposition of the 

 spicules of the vertical rows in small bundles instead of in a single series. Whether 

 the spicules in Lictyophyton were arranged singly or in bundles is not known, as no 

 spicular structure has been retained. 



Plectoderma scitulum, Hinde, n. sp. (Plate XXXI. figs. 1, 1 a, 1 b.) 



The examples of the species occur on the fractured surfaces of a soft micaceous 

 shaly rock. The vertical rows are about 7'5 mm. apart from each other; they 

 consist of a variable number, generally from five to ten, of the spicular axes in pretty 

 close juxtaposition. The spicules vary in size, from slender forms not more than 

 ■U'J mm. in thickness, to large spicules with rays 'oo mm. in thickness and 6-5 mm. 

 in length. The spicules are now mostly represented by empty moulds, and at first 

 sight appear as so many monaxial spicules crossing each other at right angles ; but in 

 places where they are less thickly grouped together, the four rays, springing from a 

 common centre, can be clearly seen, and at the point of junction there is often a 

 small circular hole, indicating a fifth ray, extending inwards at right angles to the 

 other four. The spicular rays are straight, curved, or occasionally wavy ; they usually 

 taper from the central node. In some instances fragments of the original siliceous 

 spicules yet remain, but they are so far decayed as readily to fall into minute 

 pieces. In some cases the central canal can be seen in these fragments, ^^'hether 

 the framework of larger spicules was held in position by a continuous membrane of 

 smaller spicules, as in the dermal layer of the Cretaceous genus CincUderma, cannot 

 be so readily determined ; but traces of smaller spicules, irregularly disposed between 



