THE LONDON CLAY— ITS FOSSILS— ITS HISTORY. 39 



Classes, those without backbones, the Invertebrata, or what are 

 commonly, though very incorrectly, called ** shell-fish," to which 

 nearly all the fossils of the Hampstead area belong, is thus 

 described by Professor Prestwich : " It would appear that although 

 a great proportion of the fossils range at intervals vertically 

 throughout the London Clay, yet their development is very 

 different in different zones, being abundant in some, and scarce 

 in others, whilst each zone is further marked by a few charac- 

 teristic species ; thus forming distinct, although nearly related 

 groups." 



The Vertebrata consist chiefly of reptiles and fishes. So 

 numerous are the remains of turtles in the London Clay of 

 Sheppey that no less than seven species have been described, 

 although only two species are now known to be living in all the 

 seas of the globe. In the class of fishes, eighty-eight species are 

 recorded from the Sheppey area and only six or seven from 

 the Hampstead district. Many of these were quite similar in 

 character to fishes now living though not the same in specific 

 detail. 



Of all the localities in the Middlesex area, Highgate, as has 

 been intimated, has proved by far the most prolific of organic 

 remains. The very large number of 180 named and described 

 species were obtained from the Archway excavation by Dr. 

 Wetherell. These were disentombed from a bed about 130 feet 

 below the level of the summit of the hill, doubtless the same bed 

 which has proved to be prolific of fossils at Hampstead but 

 probably lower than the Sheppey zone. This bed was cut into 

 in 187 1, when a sewer was being constructed along a portion 

 of the Finchley Road at Child's Hill. Mr. Caleb Evans obtained 

 sixty species from this excavation. He described the beds as 

 consisting of a yellowish clayey sand, to the depth of 12 feet, 

 with few fossils, but passing down first into a dark-grey clayey 



