OUR COMMON BRITISH FOSSILS. 



animals. Even when it has reached the adult condi- 

 tion, a lobster is so unlike what it will be when full- 

 grown, that it might be set down as belonging to 

 another genus. It is as if we knew nothing of the 

 metamorphoses of the butterfly, and therefore had 

 mistaken the caterpillar and chrysalis for animals 

 belonging to groups wide separated from the winged 

 insect. 



The young of the recent Limulus, or king-crab, 

 greatly resembles the adult Trilobites. As the king- 



V 



Fig. ^r.Prestwichia. Found 



in the " Penny-stone " nodules, Fig. 142. Larva 



Shropshire. 



of 



recent King-crab. 



Fig. 143. Larva of 

 Trilobite. 



crabs succeeded the latter in geological time, it may be 

 that it was due to the Trilobites having been "advanced 

 a stage." One genus found in the ironstone nodules 

 of Coalbrookdale, called Belinurus, more nearly re- 

 sembles one genus of the Trilobites (Trinucleus) than 

 the king-crabs of our own days. Again, the female 

 Bopyrus (Fig. 149), which parasitically attaches itself to 

 the inner surface of the carapace of the shrimp, has a 

 rude resemblance to the segmented body of some of 

 the less highly organized Trilobites. The fact of its 



