ISODICTYA. 149 



<c The dermal membrane is tliin and pellucid and the 

 rete with which it is furnished is regular, but the areas 

 differ somewhat in form, but rarely vary from that 

 of a triangle. The tension- spicula are very slender 

 and much longer than those of the rete, and they 

 require a power of about 400 linear to render their 

 forms distinct to the eye. The biharnate retentive 

 spicula are very few in number, and from their slender 

 proportions are not readily to be detected in situ; an 

 average-sized one measured 5-^-5 inch in length. The 

 bidentate equi-anchorate spicula are by far the most 

 numerous of all the retentive forms ; they are rather 

 irregularly distributed on the dermal membrane, some- 

 times they occur in considerable numbers, while on 

 other parts they are few in number ; they are very 

 minute and slender, not exceeding y-aVo inch in length 

 and require a power of about 500 linear to render 

 their forms distinctly to the eye. The dentate- 

 palmato-equi-anchorates are very few in number, 

 their palms are short but well produced, and they 

 are of the same length as the bidentate-equi-ancho- 

 rates. 



" The skeleton is very regular and compact in its 

 structure, and in conjunction with the other parts of 

 its organisation renders this species readily distin- 

 guishable from any other nearly allied species. The 

 retentive spicula are very sparingly dispersed on the 

 interstitial membranes, and they are not readily 

 distinguished in situ amid the sarcode in which they 

 are immersed. 



"In a small portion of the basal membrane of the 

 sponge the long slender acuate tension- spicula were 

 much more numerous than on the dermal membrane, 



