PLATE XIII. 



Page 

 Fig. 1. Hindia fibrosa, Rcemer, sp. The interior o£ a fractured specimen, showing the 



radial disposition of the canals. Natural size. The sponge has been silicified, 



and the canals now appear as minute straight rods. From Silurian strata of 



the Niagara group in Perry County^ Tennessee 57 



Fig. 1 a. The same. Portion of the interior of another silicified specimen from the same 



locality. Enlarged eight times. The rods represent the infilled canals, and the 



tubercular projections on them are produced by the siliceous infilling of the inter- 

 spaces between the spicular arms of the interior skeleton. 

 Fig. 1 b. The same. Portion of the spicular skeleton of the interior of a specimen from 



Silurian strata of the Lower Helderberg group at Dalhousie, New Brunswick. 



Enlarged seventy-six times. Drawn from a transparent microscopic section by 



means of the camera lucida. 

 Y\g. 2. Siphonia tulipa, Zitt. Natural size. From the Upper Green Sand at Warminster, 



Wiltshire 64 



Fig. 2 a. The same. A vertical median section of another specimen from Warminster, 



showing the cloaca, the course of the vertical canals which open into it, and 



traces of the radial canals which extend from the surface towards the centre of 



the sponge. Natural size. 

 Fig. 2 b. The same. Transverse section of a specimen taken from the lower portion of the 



body, immediately below the cloacal tube, showing sections of the larger vertical 



canals and the smaller radial canals. Natural size. 

 Fig. 2 c. The same. Portion of the interior spicular skeleton of 2 a. Enlarged thirty-two 



times. Drawn from a transparent section. The small square to the right of 



the figure represents its natural size. 

 Fig. ;i. Sipho7ua ficus, Goldf uss. Natural size. From the Grey Chalk at Dover ... 65 

 Fig. Sa. The same. Portion of the outer surface of the same specimen, enlarged eight 



times, showing the apertures of the radial canals and traces of the spicular 



skeleton. 

 F"ig. 4. Siphonia KiJnigi, Mant. sp. Natural size. From the Upper Chalk at Flamborough, 



Yorkshire 65 



Fig. 1 a. The same. Portion of the outer surface of the same specimen, enlarged thirty- two 



times, showing the fragmentary heads of spicules of the dermal layer. 



