16 DR. J. S. BOWERBANK ON THE SPONGIAD^L. [Jan. 7, 



larger specimen ; while in T. cranium they are extremely abundant, 

 but much more minute than those of T. simillima. I found but one 

 description of gemmule in T. simillima, which was very similar in 

 form and structure to the larger form that exists in T. cranium ; and 

 in these organs we again find a considerable difference in the pro- 

 portions of the spicula of the two species. In T. cranium the por- 

 recto-ternate spiculum of the gemmule is more slender in its general 

 proportions, and the radii are expanded at a greater angle and are 

 much longer than those of T. simillima. The unihamate spicula 

 also exhibit a characteristic variation in form. In T. cranium the 

 hamate apex is more clavate but the hook less produced than in T. 

 simillima ; in the latter the hook is not only very strongly produced, 

 but the spiculum often becomes bihamate ; and a practised eye would 

 readily distinguish the one species from the other by these characters 

 alone. Thus, although very closely allied in their general structure, 

 there appears to exist a sufficient permanent structural difference to 

 warrant our considering them distinct species. 



Tethea Cltitoni, Bowerbank. (Plate III.) 



Sponge spherical, slightly depressed, sessile. Surface even, smooth, 

 minutely pitted or areolated, areola very shallow. Oscula and pores 

 inconspicuous. Dermal rind thick and very solid, exterior and in- 

 terior surfaces furnished with a thick stratum of large closely 

 packed sphero-stellate spicula ; radii acutely conical ; interspaces 

 with comparatively few of the large sphero-stellate spicula, but abun- 

 dantly supplied with minute subsphero-stellate spicula with clavate 

 cylindrical radii, variable in form and size. Dermal membrane aspi- 

 culous. Skeleton — radial fasciculi polyspiculous ; fasciculi compact, 

 expanding slightly towards the dermal surface, through which their 

 distal terminations pass, to a slight extent forming external defences ; 

 spicula fusiformi-acuate, rarely cylindrical, or fusiformi-acerate, large 

 and long. Interstitial membranes — retentive spicula the same as 

 those of the dermal rind, few in number. Sarcode dense. 



Colour in the dried state light orange. 



Hub. Fremantle, Australia (Mr. G. Clifton). 



Examined in the dried state. 



The form of this sponge is that of a slightly depressed sphere. 

 Its greatest horizontal diameter is 13 lines, and its height 11 lines. 

 Its location is especiallyremarkable. It is seated on the top of a mass 

 of agglutinated sand and mussel-shells ; and, apparently feeling the 

 insecurity of its situation, it has given off from its base seven root-like 

 basal processes, two of which divide shortly after leaving the sponge 

 and proceed in different directions : the longest of these appendages 

 is 1 j inch, and its greatest diameter rather exceeding a line ; it ter- 

 minates in an irregularly formed adherent expansion about 3 lines 

 in diameter. These root-like appendages form no part of the specific 

 character of the sponge ; they are projected, in accordance with the 

 necessities of the individual, by almost every species of Tethea with 

 which I am acquainted. 



