THE CRETACEOUS SYSTEM 331 



strong spring tide will uncover a wide area of rock that 

 is usually concealed. This bed contains many interest- 

 ing fossils. Keptiles are represented by Chelonian bones 

 of large size ; the characteristic bony plates of the shell 

 or shield and the shoulder girdle are easily recognised ; 

 another reptile is represented by large jawbones with 

 pointed teeth. Sharks' teeth are rather abundant, and 

 complete the list of vertebrate fossils. The remains of 

 marine invertebrates are plentifully preserved in this 

 bed, the Cephalopods are represented by at least five 

 species of Ammonites, a Nautilus and a Baculites ; Gas- 

 teropods by Rostellites, Eriptycha, Pseudomelania, and a 

 large thick-shelled species of Pyropsis ; Lamellibranchs 

 by three species of Pecten,Pectunculus afrioanus, Protocardia 

 hillana, Trigonia elegans, Nemodon natalensis, Cardium denti- 

 culatum and Inoceramus. In this lowest bed there are 

 many logs of wood, blackened and partly silicified and 

 often bored into by Teredo, whose shells are still at the 

 end of the holes made by their former inhabitants. 

 Many of these fossils are much waterworn, and their 

 surfaces are in consequence abraded. The more delicate 

 shells are rarely or never found in a perfect condition, and 

 a considerable part of the rock is made up of fragments 

 of various kinds of shells. These facts, together with 

 the presence of pebbles of grits, sandstone and dark- 

 coloured slates, undoubtedly point to the bed having been 

 formed in shallow water, at the bottom of which the 

 pebbles and shells were rolled about until they were 

 covered up by the overlying deposit. The absence of the 

 thin-shelled easily-broken fossils, such as Hemiaster and 

 Cassidulus, two echinoderms that are abundant in the 



