138 SILICEOUS SPONGES. 



PLOCOSCTPniA ELEGANS, T. Smith, sp. 



1848. Brachiolites elegans, T. Smith, Ann. & Mag. Nat. Hist. vol. i. p. 355, t. 15. f. 4. 

 1878. Brachiolites elegans, Quenst. Pctref. Bd. 5, p. 490, t. 138. f. 15. 



Sponges with a subspherical body, supported on a slender cylindrical stem, termi- 

 nating in divergent rootlets. From the summit of the body an upright cylindrical 

 tube extends. The body of an average specimen is 47 mm. in height by 32 in width ; 

 the cylindrical tube measures 46 mm. in length by 23 in width ; and the stem is 

 43 mm. long. The outer surface of the body presents rounded sinuous elevations 

 and depressions ; that of the tube is smooth and even. 



The body is composed of anastomosing folds of wall about 1 mm. in thickness ; the 

 tube appears to be hollow, and formed of a simple extension upwards of the body- 

 walls. The dermal layer appears to have been furnished with minute circular canal- 

 apertures, the spicular mesh is regular, and the distance between the nodes is "35 mm. 

 The characters of the spicular nodes have been obliterated. 



The peculiar form of this species readily distinguishes it from any other of the 

 genus. T. Smith's figui-e has been drawn from two separate specimens, and is, to a 

 certain extent, a restoration of the originals. 



Distribution. Upper Chalk : Brighton, Sussex ; Hollingbourne, Birling, near Maid- 

 stone, Kent {coll. T. Smith). 



Genus TREMABOLITES, Zitt. 1877. 



Tremabolites pekfor.\tus, T. Smith, sp. 

 1848. Cephalites perforatus, T. Smith, Ann. & Mag. Nat. Hist. vol. i. t. 15. f. 2. 



Sponges subcylindrical or club-shaped, with rounded summits. The sponge is 

 composed of anastomosing, convoluted walls about 1 mm. in thickness. The summit 

 is covered with an apparently delicate surface-membrane, which is perforated by 

 several circular or oval apertures from 3 to 7 mm. in width. The spicular mesh of 

 the walls is irregular ; the nodes are octahedral, and from -4 to '5 mm. apart. 



I am unable to determine the extent of the resemblance of the interior structure 

 of this species to that of Tremabolites (Manon) megastoma, F. A. Ecem.*, but its 

 outer form, at all events, considerably differs. The forms figured by Quenstedtf 

 under the name of Cephalites pohjstoma differ from the present species in the 

 extension of the surface-membrane over the sides as well as the summit of the 

 sponge. 



Distribution. Upper Chalk : South of England {coll. T. Smith). 



* Nordd. Kreide, p. 3, t. 1. f. 9. t Petref. Bd. 5, p. 503, t. 139. f. 8-10. 



