ZOOGENESIS 



They were related to the still living king- or horse- 

 shoe crabs which still are to be found on the eastern 

 shore of North America and in the west Pacific. 



Curious and unfamiliar as are the eurypterids and 

 the trilobites, they are quite obviously crustaceans as 

 crustaceans are defined on the basis of the living 

 species. 



In the old days of sailing ships the chambered 

 nautilus was a very common ornament in the parlors 

 of the houses in the sea-port towns and cities. The 

 chambered nautilus is to be found only in the Indo- 

 Malayan seas, and there are scarcely half a dozen dif- 

 ferent kinds, all of which are very much alike. 



But in the past the nautiluses were creatures of much 

 importance as inhabitants of the sea, and about ^,500 

 different kinds have been described which have been 

 found as fossils in the rocks. In some of these the 

 shell was straight, in some it was partly straight and 

 partly coiled, in others it was loosely coiled, and in 

 many it was tightly coiled as in the still living 

 chambered nautilus. In the coiled types the coil was 

 sometimes flat — in a single plane — as in the chambered 

 nautilus, and sometimes in a spire as in a gastropod 

 or snail. 



These creatures first appeared, so the fossil record 

 tells us, in the Ordovician, in which period they were 

 found in very great variety. They were still more 

 numerous in the period just following — the Silurian. 

 They then declined, and since the time when the giant 

 reptiles were the dominating creatures on the earth 

 (the Cretaceous period) only two types persisted one 



[109] 



