Miscellaneous. 



165 



ralists, that the tuberous forms of chalk flints and chert are due 

 to organic bodies acting as nuclei, or centres of attraction, to the 

 silex of which these tubercles are composed. Mr. Parkinson, in 

 his interesting work on 'Organic Remains of a Former World' (1808, 

 vol. ii. p. 87 et seq.), had noticed acicular spicula, which he found 

 to be common to fossil sponges and fossil Alcyonia ; and in pi. 7. 

 fig. 8. of the same volume he represents the magnified appearance of 

 cruciform spines in a fossil Alcyonite resembling the Alcyonium 

 cynodium of Linnaeus, and quotes Donati as having described and 

 delineated them before him. It has also long been known that a 

 large proportion of the chalk flints in Wilts, Oxon, and Bucks, con- 

 tain, within a gray external siliceous crust of variable thickness, a 

 nucleus of semi-transparent flint, often of a purple tint, and exhibit- 

 ing distinctly a congeries of tubes and net-work, nearly allied to 

 modern Alcyonia; these Alcyonia were supposed to have acted as 

 nuclei, or centres of attraction, which became first surrounded by 

 the crust of gray flint, bearing no traces of organization, and subse- 

 quently penetrated by a kind of red or purple chalcedony, taking the 

 place of the particles of animal matter as they gradually decayed. 

 This hypothesis has been modified by Mr. Bowerbank, who has 

 superadded the agency of parasitic sponges, which he supposes to 

 have attached themselves to the alcyonic nuclei, and also to Echini 

 and other shells, forming round these organic nuclei a covering or 

 crust of sponge, which assumed, in its mode of growth, those irre- 

 gularly tuberculated forms that are so common in, and are almost 

 peculiar to, chalk flints. 



Having submitted to his microscope thin slices of chalk flints, in 

 search of Foraminifera and Xanthidia, he observed, together with 

 them, patches of brown reticulated tissue and spongiform spicula 

 pervading the entire mass of the flints under examination ; this spongi- 

 form structure was further pervaded by many tortuous cylindrical 

 and minute canals of uniform diameter, which appeared to be the in- 

 current canals of the sponge, and by other orifices of greater diameter, 

 resembling excurrent canals. He thinks that the mode in which the 

 spicula, foraminifers, and other extraneous bodies are equably dis- 

 persed throughout the silex, shows that these bodies were entangled 

 in the spongiform tissue in which their fossilization has taken place. 



With respect to the Echini and other shells, which are more or 

 less filled with, or surrounded by gray flint, he supposes the para- 

 sitic sponges to have grown both around and within the cavity of 

 these shells, and in the case of Echini to have sometimes protruded 

 outwards, sending forth branches through their orifices from the 

 parasitic sponge within. He cites the parasitic habit of some modern 

 sponges, which are found investing shells and other substances, in 

 support of this hypothesis. 



In chalk flints from Wiltshire he found the spongiform structure 

 and spicula pervading the gray crust that enclosed many zoophytic 

 nuclei ; but within these nuclei were neither spicula nor any of the 

 minute extraneous bodies which are frequent in the tubular spongi- 

 form crust The character of these fossil sponges differs from tha 

 of any recent sponge. 



