210 A MONOGKAPH OP THE 



Colour. — Alive, pale flesh-red ; dried, dull buff-yellow. 

 Habitat. — Tenby, on oyster-shells and stones. 

 Examined. — Alive and dried. 



This species is by no means uncommon on the shells of 

 the large oysters dredged at Tenby, frequently extending 

 over the whole surface of the valve, but rarely exceeding 

 three or four lines in thickness. The oscula in many of 

 the living sponges were not visible, but in some of the dried 

 specimens they were readily seen by the assistance of a lens 

 of two inches focus ; the largest observed did not exceed 

 about a line in diameter. In the dermal membrane there 

 is a manifest tendency to fasciculation of the tension spicnla, 

 but there are no symptoms of order or arrangement of these 

 loose bundles. In both the skeleton and tension spicula 

 there is a constant tendency to sub-clavation of the base of 

 the spiculum, and perhaps more especially among the tension 

 ones ; but in none of them is the clavate termination strongly 

 produced : but this tendency to dilatation of the base of 

 the spiculum is decidedly the rule, and the occurrence of 

 purely acuate forms the exception. 



The retentive spicula are abimdant, and very characteristic. 

 The bihamate ones are large and very numerous ; in a fully 

 developed state they measured ^ inch in length. The 

 bidentate anchorate ones are also rather numerous, but 

 exceedingly minute ; one of the largest of them measured 

 j^s inch in length. They are perfectly invisible in the 

 fresh tissues, and are rarely to be detected in situ, even by 

 the aid of Canada balsam, and it is only by preparation in 

 boiling nitric acid, and with a microscopic power of about 

 800 linear, that they can be satisfactorily defined. The 

 occurrence of the dentato-palmate inequi-anchorate ones is 

 rather rare. A fine specimen which I measured was nW 

 inch in length. 



