104 



GEOLOGICAL BIOLOGY. 



ably represent in only the most meagre way the forms of 

 these classes which lived in past ages. Figures 21, 22. 



FIG. 22. Arthropod. A, Pterygotus anglicus Ag. Devonian. Dorsal view. B, Pt. osiliensis 

 Schm. Silurian. Under side of head. k = cephalic shield ; r thorax ; .$ = abdomen ; a = 

 eyes ; o eyelets ; f\-f* = cephalic appendages ; 1-6 = thoracic segments ; 7-13 = abdom- 

 inal segments; t = terminal segment or telson ; e/> = epistoma ; kl = masticating plates of the 

 sixth pair of appendages ; m = metastoma ; z = median plates ; = median suture. 



Molluscoidea. The Molluscoidea, including the Polyzoa 

 and the Brachiopoda, is a group of much interest to the Pal- 

 eontologist. The Brachiopods are well preserved, and are, 

 perhaps, from the point of view of the scientific paleontolo- 

 gist, the most important group of animals he is able to study. 

 Of their history, the record is more complete, the condition, 

 as a whole, more perfectly preserved, the missing links fewer, 

 than for any other group. They have been studied more 

 thoroughly, are of greater value as marking geological hori- 

 zons, probably/than any other. They develop a chitonous 

 'or calcareous bivalved shell, the external and internal form of 



