770 DR. J. S. BOWERBANK ON THE SPONGIAD^. [Nov. 21, 



appear to be Australian. I have other specimens in my possession of 

 very much greater dimensions, and among them some rather exceeding 

 3 feet in length, having plates exceeding 5 inches in diameter ; and 

 one of the largest, at about the middle of the series of plates, 

 divides, and each stem supports a separate series of plates. The 

 plates of these sponges are not projected from the pedicles at right 

 angles to them ; they always assume a more or less ascending 

 direction, so that they form an ascending series of rather irregularly 

 shaped shallow cups. 



The dermal rete is rather more strongly produced than the 

 skeleton-structures beneath it, and it is quite irregular in form. 

 Very little of the dermal membrane remains ; the small fragments 

 that were apparent were furnished with a few dispersed spicula. 



The skeleton-structure is rather slender and delicate. The primary 

 lines vary to some extent in their diameter ; and in some there are a 

 greater number of spicula than in others. The secondary lines of 

 the skeleton are also variable in the number of their spicula and in 

 their mode of disposition. 



These singularly formed sponges are interesting, not only on their 

 own account, but also for affording us an explanation of a remarkable 

 series of fossils that are not unfrequently found in a more or less 

 perfect condition in our diluvial gravel, and which have been con- 

 jectured by some geologists to have been allied to the Trilobites, 

 while others have believed them to have been Pennatulse. 



I had longitudinal sections made and polished of several of these 

 fossils ; and on examining them by direct light with a power of 100 

 linear I found in some of them traces of sponge-structure, but so 

 indistinct as to afford very unsatisfactory evidence of their real 

 nature ; but in some, as in the specimen represented by figure 4, Plate 

 LXXIX., I found unmistakable proofs of a central axial column ; 

 and in this specimen the included sponge was of a nut-brown colour, 

 while the enveloping one was of a milk-white ; and the two were 

 cemented together by an intervening thin stratum of semitransparent 

 silex, without any indication of sponge-tissue in it. This peculiarity 

 is strictly in accordance with the natural laws of the Spongiadse, as 

 when two living specimens of the same species touch each other, they 

 unite and become as one sponge ; but, however closely two specimens 

 of different species of sponge may envelop each other, they never 

 unite and become one inseparable mass, however closely they may 

 be allied to each other in anatomical structure. 



In each section of the brown verticillate fossil sponge represented 

 by figure 4, when viewed by direct light with a power of 175 linear, 

 there were unmistakable evidences of a very delicate spongeous reti- 

 culation ; while in the white enveloping sponge traces of a very 

 different character of reticulate spongeous structure could be dis- 

 tinctly seen between the leaves of the verticillate sponge. In the 

 latter sponge there are numerous specimens of foraminated shells of 

 various species, such as are frequently found embedded in recent 

 sponges of a similarly complicated structure. The enveloping 

 sponge does not appear to be always of the same species as the 



