204 A MONOGRAPH OF THE 



dermal membrane, so as to give it the appearance to the eye 

 of having a tolerably thick dermis, while in H. carnosa there 

 is no such perceptible thickening at that part, and, in fact, 

 in neither of the species is the dermal membrane readily 

 apparent, even beneath a power of 160 linear, when sec- 

 tioned at right angles to its surface. 



On dividing three specimens of II carnosa, which had 

 been preserved when alive in a saturated solution of bay 

 salt, I could not with the unassisted eye perceive any of the 

 canals, while in a similar selection of specimens of II suberea 

 they were numerous and distinctly visible. 



It is necessary to be thus particular in the description of 

 the differential character of the two species, as in other re- 

 spects they agree remarkably in their structure ; thus the 

 form and disposition of the spicula of the skeletons in 

 II. carnosa and II suberea are scarcely distinguishable, and 

 the external form and general texture are frequently very 

 like each other. 



In many of the species of Hymeniacidon the tension 

 spicula of the membranes are of the same form and length as 

 those of the skeleton, but very much more slender in their 

 proportions; the same difference appears to prevail in those of 

 II carnosa. The stout or skeleton spicula vary greatly 

 in length, but they have their globular bases usually well 

 developed and at the extreme end of the shaft ; but occa- 

 sionally, though rarely, the globular base is placed at about 

 one or two diameters of the shaft beyond its extreme 

 end, which then terminates hemispherically, and the spicu- 

 lum becomes enormi-spinulate ; but in the slender or 

 what we may term by analogy the spicula of the interstitial 

 membranes the development of the base is by no means 

 so uniform ; sometimes half of it only is produced laterally, 

 or it terminates the spiculum in a reversed cup form, and in 

 some cases a secondary bulb is partially formed immedi- 

 ately beneath the primary one, and both are frequently 

 more or less imperfect. These variations and malformations, 

 which arise apparently from imperfect development, prevail 

 more or less also in many specimens of II suberea that I 

 have seen. 



