8 Early Irilobites of the Catnbrian ^ocks— Matthew. 
family ( PtychoparidEe ) is perhaps more instructive than in any of 
the others, as in its earlier stages the young Ptychoparian passed 
through a metamorphosis which reminds one of the adult trilo- 
bites of Group, i, especially theAgnosti; and later, in the early 
larval stages, recalls the more primitive adult forms of its own 
group. 
The embryonic forms of these trilobites are distinguished 
from the larval by the approximation of the posterior glabellar 
furrows to each other and to the occipital furrow, as well as 
by the narrowness of the occipital ring; the anterior of the 
three glabellar furrows is also very distinct at this period and 
distant from the others; and the whole glabellar ridge is narrow 
behind this furrow^, but enlarged in front. All these features 
are to be found in the Limbati among the Agnosti, in their 
adult condition. The Ptj^choparinre in their embryonic stages 
possess few' or no thoracic joint, and are typified by the Agnosti 
in their adult condition, *■ 
Of the Ptychoparians the genus Agraulos presents the most 
simple form of head shield. The dorsal suture is more direct 
and nearer the margin of the head shield than in the others 
and the surface of the shield is more even, descending all around 
to the margin. These features can be seen to be presented w^ith 
more or less distinctness in the early larval stages of the other 
Ptychoparians, whether the individual in its later stages de- 
velops into a Liostracus, a Ptychoparia, or a Solenopleura. 
Passing to the next group (Group 4) w'e meet with species 
which by their great size attracted notice at a very early period, 
and are regarded by all palteontologists as peculiarly character- 
istic of the lower part of the Cambrian system. The Paradox- 
ides differ not only in their adult condition, but also in their 
stages of development from the other trilobites of this series 
(the Acadian). But though in their structural modification and 
their development they differ from most of the others, they re- 
tained throughout life the general outline of the embryonic 
form of the glabella; only in the Regii among the Agnosti and 
in Ctenocephalus among the Conocoryphinse, and less perfectly 
in Ellipsocephalus of Group 3, do we see the remains of the 
enlarged anterior end of the glabellar ridge of the embryo; but 
in all the Paradoxides this feature is very distinctly preserved. 
