174 A MONOGRAPH OF THE 



the other. The general anatomical characters of the inter- 

 stitial structures also harmonise in a remarkable manner ; 

 and the spicula of the skeletons are of the same form ; 

 but there is one important difference which is decisive in 

 the separation of the species, and that is the great differ- 

 ence existing in their proportions, the spicula of H. mam- 

 meata, being nearly twice the length of those of H. con- 

 similis ; the former being to the latter as seven to four; 

 an amount of difference that is never found to exist in the 

 organs of any two specimens of the same species, how- 

 ever they may vary in size, age, or mode of development. 

 The hispidation of the surface of //. mammeata also serves 

 as an essential differential character. Mr. Norman has in- 

 formed me that he found this sponge living " on the upper 

 surface of rocks between tide marks near low-water at 

 Belgrave Bay, Guernsey, and at Herm, and that it lives 

 in places exposed to the full light and rays of the sun.'' 



15. Hymeniacidon variantia, Bowerbanh. 



Sponge. Massive ; surface rugged. Oscula simple, dis- 

 persed, numerous. Pores inconspicuous. Dermal 

 membrane pellucid, spiculous ; tension spicula acu- 

 ate, minute, fasciculated, retentive spicula, bihamate, 

 simple, and contort, large, and small. Skeleton. 

 Spicula, acuate, elongate, often flexuous, occasionally 

 short and flecto- or inflato-acuate, rarely acerate or 

 cylindrical. Interstitial membranes ; tension spicula 

 acuate, stout, few in numbers and acuate, very minute 

 and slender, numerous, and most abundantly with 

 fasciculi of minute acerate, and large and small 

 bihamate spicula. Sarcode abundant. 



Colour. Alive and dried, light gray. 



Habitat. St. Katherine's Rock, Tenby, Mrs. Brett* 



Exam in eel D ried. 



The type specimen in its present form is eight lines in 



