HARDWICKE'S SCIENCE-GOSSIP. 



6i 



which the power of vision seems to have been distri- 

 buted among these ancient crustaceans. Doubtless, 

 this variation was the result of special conditions of 

 existence, eyes being always possessed when they 

 were required. Thus the living male Bopvnis, or 

 shrimp-parasite (fig. 20), has rudimentaiy eyes, whilst 



Undoubtedly many of the fossil Trilobites we meet 

 with in any of the above rocks, are moults,— \}i\2X is, 

 portions of the carapace thrown off after the manner 

 of the shells of lobsters and crabs. This moult ino- 

 process appears to have peeled off the external hard 

 shell in two or three pieces. Thus, the head-piece or 



Fig. 49. Paradoxides Tessiin. 



Fig. 51. HomalanotiiS. 



Fig. 52. Phacops caudains. 



Fig. 50. Acidaspis Dufresiioyi. 



the female has none ; but this is entirely due to the 

 very different habits of life of the two sexes. Trimi- 

 cleiis is abundant in the Caradoc shales of Shi'opshire. 

 Fi'om the Cambrian to the Carboniferous forma- 

 tions we find certain Trilobites peculiar to the various 

 geological systems. Thus, Paradoxides and Agnostus 

 are peculiarly Cambrian ; TriimclcJts and Asaphus 

 are almost exclusively Lower Silurian ; Phacops and 

 Calymene are markedly Upper Silurian ; Brontes and 

 Harpes are among characteristic Devonian fossils ; 

 whilst Phillipsia and Griffithsides are genera of small 

 Trilobites — the last of their race — which are peculiar 

 to the Carboniferous limestones. 



cephalic shield, is usually found alone ; the thorax, or 

 ringed part, is also abundantly found separate ; whilst 

 the pygidium, or tail, is frequently met with apart 



Fig. 53. Ogygia Buchii. 



from the others, although it is usually adhering to the 

 thoracic part. Of course, animals which have died 

 and been buried in the mud are found with all the 

 above parts adhering to each other. The carapace or 

 shell differed in its character in various species. In 



