TRILOBITES 65 
a shield like the shell of a crab. Appendages 
numerous. ‘Two pairs of feelers. Possessing gills, 
by which they breathe. Examples: Crabs, Lob- 
sters, Crayfish, Barnacles, Wood-lice, Water Fleas, 
and Shrimps. Trilobites, so called because they 
are three-lobed, became extinct very long ago. 
Fossil remains of them are found in Cambrian 
deposits, and occur from that Period up to Per- 
Saat mian strata. These ani- 
Pas mals were in some cases 
nearly two feet long, 
but others were small. 
VEE 
Paradoxides Davidis. Conocoryphe Lyelliz. 
Fic. 15.—TRILOBITES OF CAMBRIAN AGE. 
They probably lived in great numbers on muddy 
and sandy sea-floors, as their fossils are common 
in some shales and sandstones. The Jurassic 
Crustacean known as Scapheus ancylochelrs — I 
know of no English name for it—is said to be 
a step in the direction of living crabs, but an 
older set of Crustaceans, known as Hurypterids 
(Greek, ewrys= broad, and pteron = fin), constituted 
an earlier stage of development between the trilobites 
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