THE SEVERX AND ITS TRIBUTARIES. 619 



first is that calcareous spicules, which, being more amenable to 

 solution, furnish a much more delicate test, do not occur in the 

 alluvium, or, if so, very rarely, and then as broken and corroded 

 fragments * ; in the recent ooze, on the other hand, as on the mud- 

 banks at Pownham Ferry, they are far from uncommon, frequently 

 entire, even to the points, and always colourless, transparent, and 

 glassy, as though only just shed from a decaying sponge. The 

 next fact is, that at Portishead some spicules very similar to those 

 of Isodictya cinerea were found in the sample of mud I had from 

 that place, still closely associated, being entangled together in some 

 kind of organic matter ; a fact easily explained if they had only recently 

 been washed away from a dead sponge, but well-nigh impossible of 

 belief if we imagine them to have been cashed out of the alluvium, 

 where all animal matter must, one would think, have long since 

 lost its coherence, and certainly its power of holding spicules 

 together during a rough voj T age. 



After a close examination of the recent silt and the ancient 

 alluvium, and by reason of the facts just adduced, I feel convinced 

 that the majority of the organic remains in the tidal waters have 

 been brought from a different source from that offered by the alluvium. 

 This is a conclusion to which, on general grounds, one would natu- 

 rally incline ; for the alluvium has, according to all evidence, been 

 formed under just such conditions as prevail in the estuary at the 

 present day, making some allowance for slight differences in level ; 

 and the presence of spicules and other organic debris in it cannot 

 be accounted for in a different manner from that adopted for the 

 more recent deposits. 



Having estimated the influence of the alluvium, we next turn to 

 the third possible, and the chief, sources of the organisms in the 

 modern silt ; this in all probability will likewise have been the 

 source of the fossils in the alluvium. 



So far as can be determined from a careful examination of tiie 

 coast, sponges do not grow anywhere so near Bristol on this side 

 of the Channel as Portishead and Weston ; Lynton, which is about 

 60 miles away, is the nearest possible locality ; while Ilfracombe, 

 about 15 miles further west, is well known as a rich collecting- 

 ground for both siliceous and calcareous sponges and a host of other 

 marine forms, including Sea Urchins and Starfish, which might well 

 furnish the Echinoderm network and spines so frequent in the ooze. 

 On the other side of the Channel one would need to go to Bridgend 

 before meeting with much in the way of shore life, and I doubt, 

 after a hasty visit to that localit} T , whether much would be found 

 there ; a good deal farther west is Tenby, and no naturalist needs to 

 be informed of the luxuriant growth of all kinds of marine animals, 

 including sponges, to be met with there. 



* The strength of this argument is impaired by the fact that since writing 

 the paragraph, I have found in the superficial alluvium near Weston-super- 

 Mare, one or two triradiate calcareous spicules scarcely touched by solution. 

 The question of the solubility of calcareous spicules evidently requires further 

 investigation. 



