138 BRITISH SPONGlADiE. 



as those of the skeleton, but wliile 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 j^-q inch in length, and its greatest diameter 



was 46V5 ^^ch- 



" The bidentate, equi-anchorate, 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 xT&o iiich in 

 length." 



45. IsoDiCTrA FUCORUM (Johnston), n, 322 ; iii, PI. LVI, 



figs. 16—19. 



Habitat. — Dredged off the Durham Coast and off 

 Saints Bay, Guernsey (N.). 



46. IsoDiCTYA Aldeei, Bow., II, 323 ; iii, PI. LVI, 



figs. 20—26. 



47. IsODiOTYA Bdwaedii, Bow., ii, 325 ; iii, PI. LVIII, 



figs. 15—18. 



Ilahitat. — Three fragments of massive form, and 



