SUSSEX AND PETWORTH MARBLE. 301 



in Westminster Abbey, are of Purbeck marble ; 

 in other words, they are composed of the petrified 

 shells of snails, that lived and died in the rivers 

 which flowed through the country inhabited by the 

 extinct colossal reptiles. The dark masses and veins 

 so common in these marbles, are the remains of 

 the bodies of the mollusca, changed into the car- 

 bonaceous substance termed molluskite (see ante, 

 p. 248). The shells which were empty at the 

 period of their becoming imbedded, had their 

 cavities filled with the mud, silt, &c. which are 

 now clay, marl, and limestone ; but in the shells 

 containing the animals, the gelatinous parts were 

 converted into molluskite. In polished sections 

 of the marbles this substance appears either 

 within the shells, or in black or dark brown 

 masses, and veins : the most beautiful slabs of 

 Sussex marble owe their variegated markings to 

 the contrast produced by the black molluskite with 

 the white calcareous spar. 



Potamides. — Some species of the elongated, 

 spiral, freshwater univalves, termed Potamides, of 

 which several occur in profusion in the strata at 

 Headon Hill (see PL II. figs. 2, 8, 9), are found 

 in the Wealden. A very characteristic shell, the 

 Potamides carbonarius, is figured in PL VI. fig. 5 : 

 this species was first discovered in the weald clay 

 s 2 



