Early Trilobites of the Cambrian ^ocks — Matthew. 7 
number in the rhachis of Agnostus. Each of these rings has 
an individual character, and we cannot doubt indicated features 
in the economy of this genus different from Microdiscus or any 
other trilobite of the fauna in which Agnostus is so characteris- 
tic an element. 
In Group 2 (the Conocoryphinaj) are trilobites in which one 
cannot fail to see a great advance in development beyond those 
above described. Here a facial suture is introduced, and pro- 
tective spines at the genal angles. The thorax shows greater 
mobility in its parts and receives joint after joint at the distal 
end until a long body is built up, capable of folding upon itself 
beneath the head shield and within the protection of the genal 
spines. 
Besides the provision for extending the length of the thorax, 
the head shield in this group of trilobites undergoes a great 
change of form during its growth. The change of form in the 
proportion of the parts of the head shield and the continuously 
growing thorax indicate the appearance of powers of meta- 
morphosis in this group not possessed by the two genera above 
described, and ally the Conocoryphinaj to the succeeding 
groups. 
The trilobites of Group 3, include genera which by their 
persistence in time have become characteristic of the Cambrian 
system as a whole, and in this the group differs from the pre- 
ceding one, which is eminently characteristic of the Lower 
Paradoxides beds. As the trilobites of Group 3 extended so 
far onward in time, so were they the first (so far as our know- 
ledge goes) to invade the shallow bays in which the earlier beds 
of the Acadian series were deposited. But the pioneer species 
of this group by their compact forms and thick test, seem to 
have been specially adapted to live in the shallow margin of the 
Cambrian sea, and so are the first trilobites observable in the 
barren sandstones of Band b. But they are not for this reason 
more ancient than the simpler forms of Group i, which pre- 
ferring deeper water and a muddy bottom may have existed 
simultaneously in the neighboring ocean. 
By a comparison of the embryonic and larval forms of the 
trilobites of Group 3, the close family connection of the genera 
forming it is manifest. The development of the young in this 
