ANIMAL STUDIES 



But certain subdivisions of a principal group or class of 

 animals often appear in an early epoch, become very abun- 

 dant and highly specialized in a later one, and almost 

 wholly or even totally disappear in a still later one. For ex- 

 ample, a group of certain curious animals called Trilobites 

 (Fig. 250), belonging to the great class Crustacea (which 

 includes the crabs, crayfishes, and lobsters), first appeared in 



F<G. 250. Cambrian Trilobites. a, Paradoxides Bohemicus, reduced in size ; 5, Ettip- 

 s cephcdus Hqffl ; c, Sao hirsuta ; d, Conocoryphe Sultzeri (all the above, together 

 with Pig. g, are from the Upper Cambrian or "primordial zone" of Bohemia); 

 e, head-shield of Dikellocephalus Celticus, from the Lingula flags of Wales; 

 /, head-shield of Conocoryphe Matthewi, from the Upper Cambrian (Acadian 

 group) of New Brunswick ; g, Agnostus rex, Bohemia ; h, tail-shield of Diketto- 

 cephalus Minnesotensis, from the Upper Cambrian (Potsdam sandstone) of Minne- 

 sota. From Nicholson, after BAKKANDE, DAWSON, SALTEK, and DALE OWEN. 



the Cambrian era, became very abundantly represented in the 

 Silurian era, began to decline in the Devonian, and became 

 extinct in the Carboniferous era. This was not the extinc- 

 tion of a single kind or species of animal, but of a large 



