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Show 1873.] DR. J. S. BOWERBANK ON THE SPONGIADcE. 15 numerous. They vary to a considerable extent in the degree of development 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 surrounding 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. Gemmules lenticular, surface smooth, tough, and strong; furnished with fusiformi-acerate attenuated unihamate or occasionally bihamate, and with short slender porrecto-ternate spicula mixed in fasciculi 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 distinguished. 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 appearance 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 |