448 Mr. R. Kirkpatrick on the Structure of 



chambers, each system grouped round its central and circum- 

 ambient chamber. Young specimens of Laheclna conferta 

 showed in the earliest stage a thin disk formed of forty or 

 fifty spiral coils. Later the gradually ascending spiral led to 

 tiie formation of a solid cone standing on its apex, and still 

 later to the great hemispiierical masses commonly met with. 



Orbitolites complanata, Lamarck, showing central and circumambient 

 chamber and succeeding spirals divided into segments. X 35. 

 (From Carpenter, ' Challenger ' Report on Orbitolites, pi. vi. fig. 2.) 



Fiff. 2. 



Lahechia conferta. Silurian, Gothland. Young specimens : side view 

 and view of under surface ; spiral growth indicated. Natural size. 



The extension in the horizontal plane partly takes place by 

 the formation of a central chamber in the course of some 

 particular coil, but mainly by the widening out of the spiral 



