PLATE LXII. 161 



largest of these spicula was -^B inc h> and its greatest 

 thickness at the middle of the shaft did not exceed 



25000 



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. Spongiadse.' 



In the ' Report of the British Association ' for 1868, 



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



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



macidon Jeffrey sii aiTd 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 are 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 



