90 Mr. George J. Hinde’s Description of some 
concave or occasionally flattened base, and relatively thin walls 
(Pl. I., figs. 22-26). 
P. arrecta is conical or pillar-shaped, the base concave, with 
thin margins (PI. IL, figs. 27-28). 
Up to the time of writing I have obtained from my Croydon 
garden 683 specimens of Porosphara; 624 of these belong to 
P, glubularis, 32 to P. nuciformis, 24 to P. pileolus, and 3 to 
P. patelliformis. The smallest specimen found is only 4 mm., 
whilst the largest is 27 mm. in diameter. The Chalk of this 
locality has hitherto been included in the zone of Micraster cor- 
anguinum, but the comparatively large size of many of these 
sponges and the occurrence in the same area of Offaster pilluia, 
Lam., indicate the possibility that it may be in the next higher 
zone of Marsupites. This supposition is strengthened by the late 
discovery of Marsupites and Uintacrinus in the Chalk at Bed- 
dington.* 
Specimens of Porosphwra may be found sparingly in most 
exposures of the Chalk in Croydon, when carefully searched for, 
and I have picked them up in fields, more particularly round the 
base of Croham Hurst, where the Chalk is near the surface. 
They are, however, more numerous and more readily met with 
in the Chalk cliffs at Margate, Dover, Newhaven, near Brighton, 
the Isle of Wight, and in Dorset; also near Flamborough, York- 
shire. Specimens obtained direct from the Chalk are, as might 
be expected, in a better state of preservation than those which 
have been weathered out on the surface of fields. 
Recent calcisponges with a skeleton of fused spicules, like that 
of Porosphara, were unknown till 1892, when Prof. Dr. Déderlein, 
of Strassburg, announced the discovery of a sponge of this cha- 
racter from the Japanese Sea, to which he gave the name of 
Petrostroma Schulzei. The full description and figures which 
appeared five years latert distinctly showed a close resemblance 
in structural characters to Porosphera, and I was enabled to con- 
firm this by an examination of a fragment of the recent sponge, 
kindly given to me by Dr. Déderlein. 
Shortly after, in 1898, I received from Mr. T. S. Hall, M.A., 
of the University of Melbourne, Australia, a small insignificant- 
looking fossil from beds of Tertiary age near Geelong, which he 
thought might be some sort of a sponge. To my great surprise 
it was a calcisponge with a spicular structure similar to that of 
the Chalk Porosphara and the recent Petrostroma from the Japan 
Sea. It was so beautifully preserved that the details of the 
skeleton-mesh could be seen as distinctly as in recent specimens, 
and I made it the type of a new genus, Plectroninia.} 
* Geological Mag., dec. v. vol. i. 1904, p. 482. 
+ Zoolog. Jahrbuch, Bd. 10, 1897, p. 15. 
+ Quart. Journ. Geol. Soc., vol. 56, 1900, p. 51. 
