244 A MONOGRAPH OF THE 



decidedly sub-fusiform ; and there is a general tendency to 

 flexuousness. The tension spicula of the interstitial mem- 

 branes are acuate, and very much more slender and flexu- 

 ous than those of the skeleton. The retentive spicula are 

 minute, they require a linear power of about 600 to define 

 their proportions well; the dentse are long, and their 

 apices are slightly exflected, and they mostly appear to be 

 attached firmly to the membranes by the middle of the 

 back of the shaft of the spiculum. 



The specimen described covers the surface of one valve 

 of Pecten opercularis for about a quarter of an inch in 

 thickness ; it formed part of the collection of the late Mr. 

 William Thompson, of Belfast ; it is now in the museum of 

 that town, and I have dedicated the species to him, in 

 commemoration of the valuable services he has rendered to 

 science by his ardent pursuit of natural history, and the 

 liberality with which he always placed both his specimens 

 and his knowledge at the service of his brother natu- 

 ralists. 



11. Halichondria FORCiPis, Bowerbank. 



Sponge. Massive, sessile. Surface uneven, smooth. Os- 

 cula simple, dispersed, rather small. Pores incon- 

 spicuous. Dermal membrane spiculous. Tension 

 spicula sub-clavated cylindrical, dispersed or congre- 

 gated in loose fasciculi ; retentive spiculi bi- and tri- 

 dentate equi-anchorate, large and stout, and bidentate 

 equi-anchorate spicula very minute, also bihamate, 

 simple and contort, very minute and numerous. 

 Skeleton. Rete multispiculous. Spicula acuate, stout, 

 and rather long. Interstitial membranes abundantly 

 and variously spiculous. Tension spicula sub-clavated 

 cylindrical, dispersed or congregated in loose fasciculi, 

 also forcepiform spicula entirely incipiently spined, 

 shafts of equal or unequal lengths, dispersed or congre- 

 gated in fasciculi ; retentive spicula bi- and tridentate 

 equi-anchorate, large and stout, also bidentate equi- 



