PLATE LXII. 161 



largest of these spicula was Yits inch, and its greatest 

 thickness at the middle of the shaft did not exceed 



2 5000 men. 



The dermal membrane from the same piece of the 

 sponge was also abundantly furnished with the minute, 

 bihamate, retentive spicula. 



It is a singular but very characteristic fact that the 

 same forms of retentive spicula are abundant in all the 

 seven other species of British Desmacidon with which 

 I am acquainted. 



The additional information regarding the external 

 characters and the anatomical peculiarities requires an 

 amended specific character ; I therefore propose to sub- 

 stitute the following one in place of that in p. 347, 

 vol. ii, 'Mon. Brit. Spongiada?.' 



In the ' Report of the British Association ' for 1 808, 

 p. 334, " On Dredging among the Shetland Islands," 

 the Rev. Mr. Norman has proposed to separate Des- 

 macidon Jeffreysii and make it the type of a new genus 

 under the designation of Oceanapia, from the similarity 

 in form of the bulbous mass of the sponge to a Swede 

 turnip ; but unfortunately for the propriety of this 

 character as the foundation of a genus there arc nu- 

 merous other sponges of a similar form, both British 

 and exotic, which vary so greatly in their anatomical 

 structure as to render it quite out of the question that 

 they should be grouped together in the same genus. 

 On the contrary, if anatomical similarity of structure is 

 to be assumed as the natural basis of generical arrange- 

 ment, the sponge under consideration cannot with 

 propriety be referred to any other genus than Desma- 

 cidon. The striking peculiarity of this genus is such, 

 that when a proper section of it is submitted to micro- 

 scopical examination, it is immediately referable to its 

 proper position in the generic arrangement. The 

 essential character is, that the skeleton is entirely 

 composed of multispiculated keratose fibres, which in 

 all the known British species of the genus form irre- 

 gularly reticulated skeletons. 



11 



