330 A MONOGRAPH OF THE 



being formed of single spicula disposed closely adjoining 

 each other, and at various angles to the primary lines of 

 the skeleton, thus creating so much confusion as frequently 

 to simulate the structure of a Hymeniacidon, when the sec- 

 tion happens to be not quite at right angles to the surface. 

 No retentive spicula could be detected in any portions 

 of either the dermal or interstitial membranes. 



38. Isodictya Clarkei, Bowerbank, 



Sponge. Parasitical, cylindrical. Surface even, minutely 

 hispid. Oscula inconspicuous, few, dispersed. Pores 

 inconspicuous. Dermal membrane pellucid, spicu- 

 lous? tension spicula fusiformi-acuate, slender; reten- 

 tive spicula dentato-palmate inequi-anchorate, very 

 minute, few in number. Primary lines multispicu- 

 lous, stout ; secondary lines frequently multispiculous, 

 irregular; spicula fusiformi-acuate, short and stout. 

 Interstitial membranes ; tension spicula fusiformi- 

 acuate, slender. Retentive spicula dentato-palmate 

 inequi-anchorate, very minute, few in number. 



Colour. Dried, light brown. 



Habitat. North Shields, Dr. W. B. Clarke ; Dundee, 

 Mr. Gardener. 



Exam ined. D ried . 



This species is parasitical on the main stem of a Sertu- 

 laria, which it surrounds for three fourths of an inch in 

 longth, but it does not exceed two lines in diameter at any 

 part. It has unfortunately been too much washed to 

 afford us the whole of its structural characters in a satis- 

 factory manner. The spicula in the skeleton are not very 

 numerous, there being usually not more than two or three 

 together ; and the interstitial spaces are very large and 

 open in comparison with the size of the sponge. The 

 dermal membrane appears to have been entirely, or very 

 nearly entirely, destroyed ; there is a small membranous 



