348 Mr. R. Kirkpatrick on 



vertical section a second granular " basal " layer, parallel 

 with the first, occurs in the middle of the section. 



Under magnification, the polygonal openings (105 fi in 

 diameter) of the excurrent canals are seen occupying the 

 whole upper surface. 



The thin edges of the specimen allow of no room for a 

 dermal layer, but in one small recess there was a little nest of 

 loose oxeas and styles, possibly the constituents of such a 

 layer. 



The skeleton is mainly built up of stout quadriradiates, 

 usually smooth, with fused facial rays and with the sharp 

 apical ray more or less free and pointing almost vertically to 

 the surface ; occasionally the surface of the rays is slightly 

 spined or tuberculated. 



The basal layers are composed of densely packed but 

 separate small quadriradiates, varying considerably in shape 

 and size, with rays frequently spined and terminating in flat 

 expansions. 



S]>icules. — Skeletal network : length of beams formed by 

 facial rays 70-100 /-t, and 40 fx in thickness ; apical rays 

 85-200 fi in length by 15-45 //, in thickness. 



The rays of the smaller basal quadriradiates vary from 

 18 to 35 fi in length. 



Triradiates : (1) Sagittal, rays sharp-pointed, basal ray 

 1 20 x 7 /*, laterals 60 x 7 fi. 



(2) Irregular, with slender pointed unequal rays, resting 

 on the apices (tripod type). 



(3) Tuning-forks : (a) with prongs curved away from the 

 plane of the handle, with equal prongs each ending in trifid 

 points. 



(b) Prongs and handle in one plane, prongs of unequal 

 length and spined ; there are several variations in the shape 

 of form (b) . 



Lance-shaped subtylotes and styles, finely spined with 

 disk- or crook-shaped head, 200-500 x 6 it, head 5 /a, neck 3 /*,. 



Finely spined oxeotes, 200-400 x 7 fj,. 



Plectroninia Hindei constitutes the second recent Lithonine 

 sponge that has been discovered, the first being Petrostroma 

 Schulzet, Loderlein, from Japan (10, p. 15, pis. ii.-vi.). 



The new species is near to Plectroninia Halli, Hinde, 

 from Eocene beds at Geelong and Flinders. So far as can 

 be made out from crushed fragments and a broken surface of 

 the minute specimen, the skeletal framework is fairly regu- 

 larly reticulate, without presenting any radiating main beams, 

 and the facial rays of the quadriradiates are always expanded, 

 and not tapering. The chief specific differences from 



