io8 PAL/EONTOLOGY 



curve which meets the general elliptical curve at nearly 

 a right angle. The shell is thin and fragile, and smooth, 

 except for faint growth-lines. 



The number of whorls is few : on following these care- 

 fully inwards on the upper surface it is seen that only 

 the last whorl and a half (in large specimens) has a flat 

 upper surface ; inwards from thence the surface becomes 

 convex, and after another turn it plunges downwards 

 rather rapidly, forming the hollow already noted. The 



FIG. 33. PLANORBIS DISCUS, EDWARDS, OLIGOCENE. 

 (Natural size.) (After Edwards.) 



The lines of growth are too strongly marked in the upper figure. 



actual initial part of the shell is thus invisible from the 

 upper surface, but is plainly visible below, in the centre 

 of the umbilicus if the latter is free from matrix. It is 

 clear therefore that the young shell is not only very 

 different in shape from the adult (more like Natica], but 

 is a left-handed or sinistral spiral. Indeed it is open to 

 argument whether the adult shell is not still sinistral, the 

 flat surface which has been described as upper being 

 really under. This is the more likely to be the correct 



