118 DR. J. S. BOWERBANK ON THE SPONGIAD^E. [Feb. 6, 



spicula, several of them apparently belonging to siliceo-fibrous 

 sponges. The occurrence of extraneous spicula on such sponges is 

 by no means an uncommon occurrence. On the piece of the dermis 

 of my specimen I mounted for figuring in 1855 I found several ex- 

 traneous spicula, of forms different from those figured by Mr. Kent, 

 and one of the form represented in his plate by fig. 18. 



It is a good rule never to regard any spicula that may be ob- 

 tained by the dissolution of a piece of the sponge in nitric acid and 

 mounting in Canada balsam as belonging to the species, unless they 

 can be detected in situ in a thin slice of the sponge made at right 

 angles to its surface and mounted in the same material. There are 

 very few mountings of sponge-spicula from almost any species of re- 

 cent sponge in which spicula not belonging to it may not be detected ; 

 and these are derived not only from the surface of the animal, but 

 occasionally also from its interior, in which they have been imbedded 

 in an early stage of its development ; and in some genera naturally 

 given to the appropriation of extraneous materials they are frequently 

 very numerous. 



Tethea tjnca, Bowerbank. 



Sponge spherical, sessile. Surface smooth and even, minutely 

 dotted. Oscula and pores inconspicuous. Dermis thin, pellucid, 

 furnished abundantly with very minute, simple, and contorted biha- 

 mate spicula, variable in size and form. Skeleton — spicula fusiformi- 

 acerate, large, and long, and with attenuato-recurvo-ternate connect- 

 ing and defensive spicula long and slender, and rarely with porrecto- 

 ternate ones, very small and slender. Interstitial membranes — ten- 

 sion-spicula acerate, very slender, and often flexuous ; retentive 

 spicula the same as those of the dermis, very numerous. 



Colour dull dark green. 



Hab. Hammerfest, 150 fathoms (Mr. M' Andrew, 1855). 



Examined in the condition it came from the sea. 



This sponge (the only specimen of the species with which I am 

 acquainted) was dredged at Hammerfest by Mr. M'Andrew during 

 his excursion to the North Sea in 1 855, and kindly presented to me, 

 with many other interesting specimens, on his return to England. 



The texture of the sponge is very much softer and compressible 

 than that of any other species of the genus with which I am ac- 

 quainted, and its structure much more simple than is usual in other 

 nearly allied species. The surface of the sponge is even, but is very 

 minutely dotted all over by the slight projection of the extreme distal 

 points of the skeleton-fasciculi beneath the dermal membrane ; and 

 in the midst of these fasciculi we frequently find one or two of the 

 recurvo-ternate spicula projecting slightly beyond the others ; but 

 comparatively they are rather few in number. The dermal mem- 

 brane is profusely furnished with the minute, simple, and contort 

 bihamate retentive spicula, which are distributed rather evenly over 

 the whole of its inner surface. These spicula are remarkable for 

 their minute size ; the contort ones appear usually to be the largest ; 



