18/3.] DR. J. S. BOWERBANK ON THE SPONGIADtfl. 15 



numerous. They vary to a considerable extent in the degree of de- 

 velopment of their triradiate heads, the rays in some being twice 

 the length of those in others. 



The ovaria are abundant in the dermal crust ; and they are also 

 numerous on the interstitial membranes, where they are found in 

 every stage of progressive development, some of them having a 

 diameter not more than one-tenth part that of the mature ovarium ; 

 in their fully developed state they are globular with a very slight 

 amount of depression. 



Tethea simillima, Bowerbank. (Plate III.) 



Sponge globular, sessile ; surface even, strongly hispid. Dermal 

 coat abundantly furnished with stout fusiformi-acerate spicula sur- 

 rounding the defensive fasciculi. Dermal membrane thin, pellucid. 

 Oscula and pores inconspicuous. Spicula of the skeleton fusiformi- 

 acerate large, and long. Defensive spicula external, collected in 

 fasciculi ; fusiformi-acerate large and long, few in number ; fusi- 

 formi-porrecto-ternate abundant, radii short and stout ; and attenuato- 

 recurvo-ternate very abundant, shaft slender, very much attenuated. 

 Sarcode furnished sparingly with minute bihamate spicula. Gem- 

 mules lenticular, surface smooth, tough, and strong ; furnished 

 with fusiformi-acerate attenuated unihamate or occasionally biha- 

 mate, and with short slender porrecto-ternate spicula mixed in fas- 

 ciculi radiating from the centre of the gemmule. 



Colour, dried, light brown. 



Hab. South Seas (Sir Everard Home). 



Examined dried and in spirit. 



The characters of this species are given from two specimens 

 brought home from the South Seas by Sir Everard Home. The 

 most perfect specimen is in the dried state, and measures eleven lines 

 in diameter. The second one is about one third of a much larger 

 specimen, not less than two inches in height, and is in spirit; both 

 specimens are in the Museum of the Royal College of Surgeons. 



This species is remarkable for the very close resemblance it has to 

 Tethea cranium of our northern seas ; and although I have designated 

 it as a species, I have great doubt whether it should be thus distin- 

 guished. The same forms of spicula are found in both ; but their 

 comparative proportions and the degree of their prevalence in the 

 respective parts of the sponge differ to a considerable extent. There 

 is but a very little difference between the skeleton-spicula ; those of 

 T. simillima are slightly the longer and greater of the two : but in 

 the defensive spicula there is a considerable amount of discrepancy. 

 In the projection of the defensive fasciculi of T. cranium the appear- 

 ance of recurvo-ternate spicula is very rare, while in T. simillima 

 they are almost as abundant as the porrecto-ternate ones ; and in the 

 first-named species the radii of the porrecto-ternate spicula are very 

 much longer and more attenuated than in the last species named. 

 In the sarcode of the small specimen of T. simillima I could not 

 detect the minute bihamate spicula ; but I found a few in that of the 



