~~ 
982 The American Naturalist. [November, 
crustacean in a nearly per- 
fect state of preservation,and 
Figure 12 is a group of trilo- 
bites of various genera, most 
of them belonging to the 
typical genus of the Lower 
Cambrian, Olenellus. This ge- 
nus, as pointed out by Wal- 
cott, is probably genetically 
related to Paradoxides, the 
typical genus of trilobites of 
the Middle Cambrian, and it 
has its modern, living proto- 
Fig. 8. Lamellibranchiata. 
type in the common horse- 
shoe crab, Limulus, of the 
Atlantic coast. It would 
be an interesting fact, and 
a not altogether improb- 
able one, to find in Limu- 
lus a descendant of Olenel- 
lus of the LowerCambrian. 
Besides the great varie- 
ty of forms found in this 
very ancient fauna of the 
globe, there is the interest- 
ing subject of geographical 
distribution and its con- 
nection with the study 
of evolution. As already 
stated, the three great divisions of the Cambrian, the Lower, 
Fig. 9. Trails of Annelids (P/anolites). 
