70 THE ORIGIN OF MAN 



in the Jurassic except the ganoids and lung- 

 fishes. The former were now at their highest 

 development, not only in the fresh waters but in 

 the seas as well. The sharks of the seas had at- 

 tained their modern development, and in the 

 Jurassic the flat fishes known as rays made their 

 appearance. 



The seas of this period were replete with in- 

 vertebrate life. Sponges were very common and 

 crinids were at times. In the Lower Jurassic 

 are found the largest forms of crinids that ever 

 lived. Likewise sea-urchins were common, but 

 not so the starfishes and brittle-stars, and those 

 that were present were of the modern type. 

 Brachiopods were still plentiful, also the gastro- 

 pods, and the seas were full of small and large 

 bivalves, but the most characteristic shell-fish of 

 the Jurassic were the ammonids, to be described 

 later. The crustaceans are represented by many 

 kinds of lobsters and the ancestors of the mod- 

 ern crabs also appeared during this period. 



