298 PALEONTOLOGY 



exact taxonomic value, but is a very useful one, especially 

 in palaeontology. 



i. Zaphrentis (Fig. 89) is a genus of which there are 

 several species common in the Carboniferous Limestone. 

 Though the following description applies essentially to 

 Z. konincki, much of it is true of other species. 



The shape is that of a drinking-horn th'at is, a cone 

 with its axis more or less curved. An ordinary specimen 

 is about an inch (25 mm.) in height, and about 12 mm. in 

 diameter at the broad end. The external surface is 

 marked with concentric rings, obviously lines of growth, 

 and in some species there are also fine vertical ribbings.* 

 The curvature of the cone may be in part the effect of the 

 conditions of growth : a cone fixed by its apex and grow- 

 ing larger upwards is in unstable equilibrium. If not 

 firmly fixed it will topple over towards one side, and, 

 fixing itself in that position, will become curved in the 

 effort to grow straight upwards. The direction of curva- 

 ture is always determined by the internal structure to be 

 described next. 



At the broad (distal) end the cone is hollowed out into 

 a depression (the calyx) about 6 mm. deep, with steep sides 

 and a nearly flat floor. A well-marked groove, the fossula, 

 extends from the centre of this floor to that side which 



* In specimens from Tournai, Belgium, otherwise very well 

 preserved the surface ornament is largely obliterated by chemical 

 alteration of the substance of the coral. The original aragonite has 

 been replaced by chalcedonic silica, not uniformly (as is often the 

 case in other fossils), but in a series of rings, alternately thick and 

 thin. This is known as " beekite "-structure (see Chap. XL). It is 

 found also in brachiopods, lamellibranchs, etc., in strata of various 

 ages. 



