PLATE XO. 325 



are very characteristic of the genus ; but their dis- 

 position is very irregular ; their length is frequently 

 not more than that of a large spiculum, and in all cases 

 the spicula of which they are composed appear to have 

 their bases and apices coincident. 



The most remarkable character in this species is a 

 negative one. In every other species of the other 

 British or foreign species with which I am acquainted 

 the dermal membrane is furnished, more or less, with 

 beautiful rosette-shaped groups of inequi-anchorate 

 retentive spicula, and intermixed with them there are 

 frequently numerous bihamate ones ; but in this 

 species those forms appear to be entirely absent, and 

 no other form is present than the acuate ones described 

 above. Under these circumstances, this simplicity of 

 structure renders this species more difficult of dis- 

 crimination than usual ; but the fasciculated structure 

 of the skeleton, and the coincidence of the bases and 

 apices of the spicula of which they are composed 

 become the most important characters in its deter- 

 mination. 



ISODICTYA DUBIA, Bowerdank. 

 Plate XC. 



Sponge massive, sessile, nodulous. Surface minutely 

 and irregularly asperated. Oscula simple, dispersed, 

 minute. Pores inconspicuous. Dermal membrane 

 thin and pellucid, abundantly spiculous ; spicula 

 acuate, rarely acerate, same size as those of the 

 skeleton, dispersed ; retentive spicula bidentate, equi- 

 or slightly inequi-anchorate, minute, and very few in 

 number. Skeleton. Slender and much complicated ; 

 primary lines multispiculous ; secondary lines varying 

 from, multispiculous to unispiculous, numerous and 

 very irregular ; spicula acuate, rarely acerate, variable 

 in size. Interstitial membranes aspiculous ? 



Colour. In the dried state, light grey, with a tint 

 of yellow. 



