08 



bony structure, which is composed of numerous minute centres of os- 

 sification, touching each other at their extreme points, and leaving 

 numerous small spaces unoccupied by the bony structure, so that the 

 whole surface assumes a beautifully checquered appearance when ex- 

 amined as a transparent object with a power of about 100 linear. 



The sponges which I have now to describe are exceedingly anoma 

 lous in their structure, especially the first of the two, which I receiv 

 ed, with many others, from my friend Rupert Kirk, Esq., of Sydney, 

 Australia ; who procured it from the coast at Wollongong, about forty 

 miles from Sydney. Hitherto we have found that where silex has 

 been the material with which the skeleton of the sponge tribe has been 

 strengthened and supported, that earth has always been presented to 

 us as a true animal secretion, either in the shape of spicula of various 

 forms, or of very minute spherical molecules ; but in these species, 

 although in great abundance, it is decidedly of extraneous origin, 

 consisting of numerous grains of sand built into the substance of the 

 fibrous skeleton of the animal, without any definite arrangement : the 

 whole being surrounded externally by a coating of the cartilaginous 

 or horny matter of which the foundation of the skeleton is construct- 

 ed, and with which we are so familiar in the sponges of commerce. 

 We are acquainted with many instances in which extraneous substan- 

 ces are employed externally to assist as a covering or defence ; as 

 among the Mollusca, by Trochus agglutinans, which attaches to the 

 external surface of the shell the small shells and fragments of bivalves 

 or any other substance that suits its purpose. We observe also among 

 the aquatic larvae, that the cases in which they pass so great a por- 

 tion of their existence, are, in many species, most ingeniously and 

 beautifully built up of fragments of small stems of plants, bits of shells 

 and other materials ; and in Sabella Belgica, so common on our own 

 coasts, the animal resides in an elongated conical tube, which is con- 

 structed, in the most symmetrical and beautiful manner, of grains of 

 sand cemented together by a strong cartilaginous or horny matter ; 

 but in no instance that I am aware of have we found the animal ske- 

 leton formed from extraneous materials, as in this curious and new 

 form of sponge. 



Duseideia,* Johnston, MSS. 

 (Plate vi. fig. 9). 

 D. Kirkii. Sessile, massive, somewhat compressed. Skeleton 



* The following are the characters established by my friend Dr. Johnston for this 



