82 



HANDBOOK OF FOSSILS. 



urchins, such as the "sugar loaf" (Ananchytes) and the heart- 

 shaped Micrastcr, the Brachiopods or Lamp-shells ( Tercbratula, 

 RhyncJwnella), a " Thorny Oyster " (Spondylus spiuosus), be- 

 sides Ammonites, Belemnites (part of the internal shell of a 

 kind of cuttle-fish), and the teeth of several species of sharks. 

 Altogether the chalk is about 1,000 feet thick. 



b. Upper Greensand is a series of greenish-grey sands and 

 sandstones. The green colour, on close inspection, is seen to be 

 due to the presence of innumerable small green grains of a 

 mineral called glauconite. These are frequently casts of the 

 chambers of the very same fora- 

 minifera that the chalk is so largely 

 composed of. 



Nodules and layers of "chert" 

 (an impure kind of flint) oc- 

 cur in it, whilst in places it forms 

 a hard rock called "firestone." 

 The commonest fossils are Brachio- 

 pods, very similar to those in the 

 chalk, a scallop-shell with four 

 strongly marked ribs on it (Pecten 

 quodricostatus}, an oyster with a 



i R oA,;,v A frrnn^TTrmpr curved beak (Exogym colitmba), and 

 (a t5r3.cniopou. trorn tnc upper , , -^ . _. . . 



Greensand). a pear-shaped sponge (Siphoma py- 



riformis). The Upper Greensand 



is better seen at places in the southern part of the Isle of \Vight, 

 in cliffs on the Dorsetshire coast, in 

 Wiltshire, at Sidmouth, and in some 

 parts of Surrey. 



c. Ganlt, a stiff blue clay abound- 

 ing in fossils : Ammonites often re- 

 taining their pearly shell ; Belem- 

 nites, a bivalve with very deep 

 furrows on it (Inoccramus sulcatits], 

 and its first cousin (/. concentricus, 

 jp. 21), in which the ridge-like mark- 

 ings correspond with the lines of 

 growth, besides many others, may 

 be obtained in abundance from it. 

 Layers of phosphatic nodules occur 

 at irregular intervals. The gault is 

 best studied at East Wear Bay, near 

 Folkstone ; it may also be seen 

 in Dorsetshire, Wiltshire, and 





Ammonites mtritus 

 (from the Gault), 



