230 PLATE LXXVI. 



auchorate spicula cannot be distinctly defined, without 

 tlie aid of a power of 600 or 700 linear. The skeleton 

 is remarkably close and dense in its stricture, not from 

 the strength of its primary lines, but from the pro- 

 fusion of the secondary ones, Avhich are by no means 

 always at right angles to the primary lines ; but still 

 the skeleton maintains a very symmetrical appearance 

 in a section at right angles to the surface of the sponge. 

 By a very careful examination of sections in Canada 

 balsam I detected a few of each form of retentive 

 sjDicula on the interstitial membranes, but they were of 

 such rare occurrence that they might very readily 

 escape the notice of a hasty observer. The remarkable 

 structure of the skeleton and the peculiarities of the 

 spicula render the discrimination of the species in a very 

 satisfactory manner when examined in Canada balsam, 

 but without the aid of this material the dark-coloured 

 and abundant sarcode obscures nearly all the most 

 discriminative characters of the sponge. 



There are only two other British Isodictya, I. 

 infimdihuliformis and I. clissimilis, "which have the spicula 

 of the primary and secondary lines of different forms, 

 and fi'om both of them the species under considera- 

 tion is readily distinguishable. 



Raphiodesma sordida, Bowcrhank. 



Plate LXXVI. 



Sponge coating, parasitical. Surface rugged and 

 uneven. Oscula inconspicuous, simple, dispersed, 

 minute. Pores inconspicuous. Dermal membrane 

 spiculous ; tension spicula subclavate acuate, occa- 

 sionally subfusiform, of the same length and form of 

 those of the skeleton, but much more slender, dispersed, 

 rather numerous ; rarely loosely fasciculated, and a few 

 long slender tricurvate accrate ones. Retentive spicula ; 

 contoT't, biliamate, rather numerous; also bidentate, 

 inoqui-anchorate, dispersed, and dentato-palmate, in- 



