126 SILICEOUS SPONGES. 



elevated, crater-like margins. The under surfaces in all the specimens is firmly 

 attached to the rocky matrix, and its characters are not visible. The thickness of 

 the sponge is about 5'5 mm. 



The upper surface, or dermal layer, is composed of a very delicate, minutely porous, 

 siliceous network ; the spicular structure is so minute that it is not distinguish- 

 able unless under the microscope ; beneath this dermal layer the loose tissue of the 

 interior skeleton is exposed. This is formed of spicules with stout arms and lantern 

 nodes. The distance from node to node varies between -28 and 'ST mm. In some 

 places the interspaces of the mesh are filled with minute slender hexactinellid 

 spicules, apparently disposed irregularly. 



There are three examples of this species, which, so far as I am able to judge, corre- 

 spond with Eoemer's figure and meagre description. One specimen is from the Grey 

 Chalk, and is now calcareous in its composition ; the other two, from the Chalk Marl, 

 still retain in part their siliceous structure, and from these I have been able to 

 ascertain the spicular characters. 



Distribution. Chalk Marl : Ventnor. Grey Chalk : Dover. 



Ophrtstoma ocellatum, Seeley, sp. 



1873. Porospongia oceUata, Seeley, MS. See Proceedings Geol. Soc. vol. xxix. p. 68, pi. 6. 



f. 1-4. 

 1873. Ventriculites cavatus, Sollas (non T. Smith), Proe. Geol. Soc. vol. xxix. p. 68. 



Sponges forming flattened or slightly curved expansions with rounded margins. 

 The wall is from 3-5 to 5 mm. in thickness. The upper surface is perforated with 

 oval apertures from 2 to 3 mm. in diameter, and about their own diameter apart, 

 disposed in diagonal lines or irregularly. The mai-gins of these apertures do not 

 project as in tlie preceding species. The characters of the under surface are con- 

 cealed in the specimens which I have examined. 



The spicular mesh of the interior of the wall is of an open irregular character. 

 The arms are robust, with prominent lantern nodes ; the distance from node to node 

 is '4 mm. The spicular structure of the dermal layer has been obliterated. 



This species differs from the preceding in its smaller size and more regular form, also 

 in the generally smaller dimensions and regular disposition of the surface-apertures ; 

 the spicular structure of the interior is also more open, and the spicular arms are 

 larger than in 0. micrommatum. Only empty moulds of the spicular skeleton of the 

 wall are preserved ; good representations of these are given by Sollas (/. c). 



The name of this species was first given by Prof. Seeley to specimens in the 

 Cambridge Woodwardian Museum ; and it was afterwards figured by Sollas under 

 the same designation, though in his description it is placed under the name of 

 Ventriculites cavatus, T. Smith. There is, however, no alliance, beyond the common 



