1 8 OUR COMMON BRITISH FOSSILS. 



of the limestone appears friable and " rotten." This 

 is just the material to look for microzoa, for Radio- 

 larians, sponge-spicules, etc. Those of a fossil " glass- 

 rope " sponge (Hyalonema) are met with in abundance. 



Mr. James Thomson, F.G.S., discovered a bed of 

 " rotten limestone " at Cunningham Baidland, Dairy, 

 Ayrshire, and the spot is now well known to all 

 Glasgow geologists. Sponge spicules, notably those 

 of Hyalonema, are abundant, and they are found 

 converted from their original siliceous to a calcareous 

 composition. Several other places where microzoa 

 occur have been made known in Scotland ; and Mr. 

 John Young, F.G.S., has carefully examined their 

 materials and published details of his discoveries in 

 the "Transactions of the Glasgow Geological Society." 



Whenever the rock appears "rotten," or the thin 

 beds of limestone shale which are frequently " sand- 

 wiched" between the limestones seem more than 

 usually crumbly, the student should box some of the 

 material, and carefully note where he obtained it. 

 At home, with the aid of the microscope, he can 

 occupy his winter evenings in its examination, and 

 perhaps he may thus add to our scanty knowledge 

 of Palaeozoic sponges. 



Until we come to the Cretaceous rocks, we know 

 almost less of the structure of the Secondary sponges 

 than of those just mentioned. There must be remains 

 of them somewhere, and by looking out keenly for 

 rocks having a peculiar structure, which the student 



