MOLLUSC A 465 



so very common, make their first known appearance in the Jurassic, 

 but they were still rare, and connecting links between the long- 

 tailed and short-tailed series were more abundant. Isopods and 

 Stomatopods also abounded. 



The Limuloids are reduced to the single genus Limulus, which 

 then occurred in the European seas, while the living horseshoe 

 crabs of that genus are found only on the east coast of the United 

 States and in the Molucca Islands. 



Spiders and Centipedes have not yet been found, — another 

 illustration of the imperfection of the geological record. There 

 can be no doubt that these animals existed in Jurassic times, for 

 we find them both before and after that period. 



Insects, on the other hand, are found in multitudes in certain 

 localities, and display a great advance in the number of types over 

 any of the Palaeozoic periods. The Orthopters and Neuropters 

 which we found in the Palaeozoic are enriched by many new 

 forms, such as grasshoppers and dragon-flies, while beetles ( Cole- 

 opterd) become very abundant. The Hymenopters (ants, bees, 

 wasps, etc.) and the Dipters (flies) date from the Jurassic, and 

 Lepidopters (butterflies) have also been reported, though doubt- 

 fully. As the latter insects are dependent upon a flowering vege- 

 tation, definite proof of their presence in the Jura will establish 

 the existence of the Angiosperms. 



Brachiopoda. — These shells are still common in the Jura, but 

 they are simply a profusion of individuals belonging to a few 

 genera, most of which persist in our recent seas. Terebratula, 

 Waldheimia, and Rhynchonella are much the most important 

 genera, and the last stragglers of the long-lived Palaeozoic Spirit 

 ferina are here found. 



Mollusca. — The Bivalves, which had already become such im- 

 portant elements ©f the Triassic fauna, greatly increase in the Jura, 

 their shells forming great banks and strata. Many of the genera 

 are still living, and only a few of the more abundant ones can be 

 mentioned here. Oysters like Gryphcea (IX, 2), Exogyra, and 

 Ostrea itself are common. Trigonia (Fig. 151 and PI. IX, Fig. 5) 

 is especially characteristic of the Jura, but a few representatives 



