150 



Period, where, however, they are found in deposits, not only 

 indicating a warm climate but actually forming the fossilized 

 remains of coral reefs such as we find at Hackness, East 

 Ayton. and Pickering. 



The genus Astarte, according to Yon Zittel, is first definitely 

 known from the Triassic formation. It then gradually began 

 to increase in numbers, and reached its maximum growth in 

 adaptation to the warm tropical seas of later Jurassic times. 

 After this epoch it slowly but surely begins to decrease, pro- 

 bably not so much owing to the varied changes of geography 

 and climate which the genus witnessed (though these factors 

 have doubtless played some part in its diminution), but 

 perhaps owing to the competition of more vigorous molluscs 

 and animals. The pressure of these rivals may have caused 

 the present descendents of the semi-tropical Jurassic A start cs 

 to become adjusted to the colder seas of the Northern 

 Hemisphere, and that far from regarding these seas as the 

 original home of the genus, we must probably place it. so far 

 as our continent is concerned, in Central Europe in late 

 Jurassic times. The present forms, including our two local 

 species, are the relicts of a once numerous family of semi- 

 tropical shells. 



We thus see that the common shells we so carelessly crush 

 with our feet on the sea shore, have a very complex history 

 extending many millions of years into the past, and of which 

 we can only form a very indefinite idea. What has just been 

 said is applicable to every marine animal on the Cleveland 

 Coast ; for some, an imperfect picture of their evolution can 

 be formed, for many others even this imperfect picture is 

 an impossibility, since all or nearly all records of their 

 enormous past have vanished for ever. 



3. — Past Marine Faunas. 



What we have just considered concerning the geological 

 history of Astarte, naturally leads us to the third division of 

 our subject, the past marine faunas of Cleveland. Although 

 we have learned that the present marine fauna has undergone 

 many vicissitudes in its development, we have yet to see 

 that our district was occupied by many marine faunas, not 



