172 ANATOMY AND PHYSIOLOGY 



mis cmstular, furnislied abundantly with closely 

 packed ovaria. Ovaria siliceous, formed of cuneiform 

 spicula, firmly cemented together in lines radiating 

 from the centre of the ovary. • Pores furnished with 

 oesophageal tubes, terminating in the distal extremity 

 of each intermarginal cavity. Intermarginal cavities 

 symmetrical, subcyHndrical, with a pyloric valve at 

 the proximal end of each. 



Type, Pachymatisma Johnstonia^ Bowerbank. 



Since the first publication of my description of the 

 sponge on which this genus is founded in the " Synopsis 

 Spongiarum" of Dr. Johnston's 'History of British Sponges,' 

 p. 243, I have found it necessary to base the generic cha- 

 racters of the Spongiadse on the structural peculiarities of 

 the skeleton and reproductive organs. I have therefore 

 reconstructed the character of the genus in accordance with 

 this rule. 



This genus is closely alUed to Geodia in its organic 

 structure, but the difference in the arrangement of the 

 skeleton readily distinguishes them. The general aspect of 

 the species of each genus is also strikingly distinct. I am 

 acquainted with seven species of Geodia and three oiPachy- 

 matisma, and in every case the species may be readily 

 referred to its proper genus even by its general aspect. 

 AU the species of either genus have a crustular dermis, 

 and the structures of the ovaria are also, alike in each. I 

 have described the anatomical peculiarities of the latter 

 organs so fully in the description of the generic characters 

 of Geodia as to render it unnecessary to treat of them 

 under the present circumstances. Fig. 353, Plate XXVIT, 

 a view of a section at right angles to the surface from 

 Pachymatisma Johnstonia, exhibiting the irregularity of the 

 interstitial structures immediately beneath the dermal 

 crust X 50 hnear. 



