138 BRITISH SPONGIAD^E. 



as those of the skeleton, but while those of the skeleton 

 are nearly uniform in length and diameter, those of 

 the dermis, although tolerably uniform in length, vary 

 to a great extent in their diameter, many of them not 

 being half the diameter of a completely developed 

 skeleton one, many of which are intermixed with those 

 of the dermis. An average-sized skeleton-spiculum 

 measured y^-Q inch in length, and its greatest diameter 

 was -f^T5 inch. 



" The bidentate, equi-auchorate, retentive spicula are 

 very sparingly dispersed in the dermal and interstitial 

 membranes ; they are best seen in the latter in situ, as 

 in the former they are to a great extent obscured by 

 the abundance of the sarcode in which they are im- 

 mersed. They are very slender and delicate in their 

 structure, requiring a power of about 400 linear to 

 render them distinctly to the eye. They are uniform 

 in size. One of the largest measured y-sVo inch in 

 length." 



45. ISODICTYA FUCOEUM (Johnston), n, 322 ; in, PI. LVI, 



figs. 1619. 



Habitat. Dredged off the Durham Coast and off 

 Saints Bay, Guernsey (N.). 



46. ISODICTYA ALDEEI, Bow., n, 323 ; in, PI. LVI, 



figs. 2026. 



47. ISODICTYA EDWAEDII, Bow., n, 325 ; m, PI. LVIII, 



figs. 1518. 



Habitat. Three fragments of massive form, and 



