4 PALAEONTOLOGY 



of the valves at successive intervals, and it is evident 

 that the earliest formed shell must be that in the region 

 of the umbo. 



The region of the umbones is usually obscured by 

 rock substance, which it is a tedious matter to remove, 

 but when this is done some interesting features are 

 revealed. The anterior side of the circular opening 

 (foramen} is seen to be differently bounded from the 

 remaining three-quarters of its circumference. It is not 

 bounded by the ventral valve proper, but by the oblique 

 ends of two small plates of triangular shape : these are 

 called the deltidial plates, and the complete gap in the 

 shell (of which the foramen is only a part) which would 

 be exposed by their removal is called the dflthyrium. 

 On either side of these plates the surface of the ventral 

 valve shows a somewhat crescent-shaped, concave area, 

 having the appearance of being cut out from the shell : 

 this is the cardinal area. Along this area the margin of 

 the ventral valve is seen to be pressed close against and 

 slightly to overlap that of the dorsal valve : it is easy to 

 see that here the valves are hinged together, while else- 

 where they may have been free to gape apart, the gaping 

 being greatest at the anterior end. Indeed, although the 

 great majority of these fossils are found with the valves 

 tightly closed, one is occasionally found in which they 

 yawn more or less. 



All the description so far given will apply equally well 

 not only to the two forms from the Cornbrash, but to 

 many others found in older or younger strata. All these 

 were long united under the common name Tevebvatula, 



