118 THE ROYAL SOCIETY OF CANADA 



(Owen) collected in the Black River limestone at Pointe Claire, Que. 

 In this specimen the calcite of the skeleton has been replaced by 

 silica, while the limestone in which it is embedded has not. It illus- 

 trates a case of selective replacement. In places the outer surface of 

 the test shows the septa. At other places it shows roughly con- 

 centric ridges arranged around a number of centres. These are 

 developed over a distance of as much as an eighth of an inch from a 

 centre. Usually, the lines indicating the trend of the septa are 

 destroyed on the outer surface of the test where the concentric ridges 

 have been formed, but this is not always the case. 



A number of other cases have been noticed, in the McGill Uni- 

 versity collection, since the above described specimen was found. 

 A list of these will be found below. From the list it will be noticed 

 that fossils preserved in silica from the Niagara dolomitic limestone 

 and from the Trenton limestone afford the most abundant examples 

 in the McGill University colleection, and that the examples are not 

 confined to any particular group of organisms but include replaced 

 lamellibranchs, brachiopods, gasteropods, corals, stromatoporoids, 

 cephalopods and cystids. By no means every specimen preserved in 

 silica shows the development of concentric rings of ridges. Indeed, it 

 is to be especially mentioned that of the large number of examples 

 of fossils preserved in silica from the Corniferous limestone of Ontario 

 only one specimen showing the structures has been found in this collec- 

 tion. 



A comparison of these forms with the ridges developed in drying 

 gelatine seems to suggest that the cases are analogous, and just as all 

 drying gelatine does not pucker up into ridges, so all fossils preserved 

 in silica do not show them. No attempt has been made to determine 

 the causes underlying the formation of the ridges and thus explain why 

 the fossils from the Corniferous limestone usually do not show them, 

 whereas those from the Trenton and Niagara often do. There seems 

 to be every probability that the ridges are due to causes operating 

 during the loss of water of the gelatinous silica which replaced the 

 calcite of the skeletons. Whether this were due to the presence of 

 different reagents in solution, or to different concentrations of re- 

 agents present, or not, may be left an open question. 



It is evident from the partial enclosure of one set of ridges in the 

 limestone of the first specimen described that the banded structures 

 have been developed while the replaced fossil was still enclosed in the 

 rock. 



Two other examples of this structure which have come under the 

 writer's notice call for special mention. The first is a specimen of 

 Streptelasma sp. from the upper Helderberg of Columbus, Ohio. In 



