BRITISH SPONGIAD^. 95 



Esuamined. — In the dried state. 



I received six specimens and a fragment of this species 

 from the Rev. A. M. Norman, who obtained them at Shet- 

 land. They were all nearly circular, and much depressed ; 

 they varied in diameter from three to seven lines, and the 

 greatest height did not exceed three lines. In the middle 

 of the upper surface of each there is one or more slightly 

 elevated mammae, and some of them presented rather 

 indistinct indications of a closed osculum at their summits. 

 The marginal portions of the specimens were covered with 

 tine sand, which adhered strongly to the surface, but the 

 mammae, and immediately around them is clean, and free 

 from it, probably by the action of the excurrent streams of 

 water from the oscula. None of the specimens possessed 

 the natural base, and the whole of them appeared as if 

 they had been removed by a knife from the surface of a 

 flat shell or stone. In the depressed form this species 

 harmonises in habit with T. Collingsii. At the edge of the 

 largest specimen a young T. cranium, scarcely exceeding a 

 line in diameter, has fixed itself, as if to bear testimony to 

 its Shetland locality. 



The minute hispidation of the external surface is scarcely 

 visible by the aid of a lens of one inch focus. When soaked 

 in water the sponge appeared to be very firm and strong, 

 and a section of the dermis in the dried condition presented 

 very much the same texture and appearance as a slice from 

 the margin of a thin white card. ■ The terminations of the 

 skeleton fasciculi forming the external defensive system, 

 project about half or two thirds the leiiigth of a skeleton 

 spiculum beyond the dermal surface, but those of the 

 secondary system or dermal defensive spicula, rarely pene- 

 trate the dermal membrane. They are extremely numerous, 

 and very closely packed together, their apices appearing 

 immediately below the dermal membrane, and the bases of 

 the largest and longest of them reaching to the inner sur- 

 face of the coriaceous dermis. In the general character of 

 form they closely resemble the skeleton spicula, but difier 

 from them in being very much shorter, stouter, and more 



