96 G. F. MATTHEW ON ILLUSTRATIONS OF 
is now in its normal position, on a line with the front of the ocular lobe, and about half-way 
from the front of the dome, as in the full grown trilobite. 
Occipital ring. —Here there is a decided change in the narrowing of the ring and the 
straightening of its posterior margin. This part does not hereafter project behind the 
rest of the shield, as in the earlier stages. The occipital spine recognized in the younger 
individual has not been detected in this, and the maturer tests of the typical form of 
P. eteminicus. F 
Posterior margin.—This is much wider and stronger than before, the gain being chiefly 
in the width of the the furrow. 
Sculpture—The band of minute tubercles, which in the 10.2 mm. size can hardly be dis- 
cerned with a glass, at this stage becomes quite distinct ; and also the extension of it which 
passes along the posterior furrow. The tubercles on the axial line are also now visible to the 
unassisted eye. The granulations on the surface of the test are now everywhere distinctly 
seen, as are also the raised lines on the front of the dome and toward the end of the 
anterior marginal fold on its inner and outer margins. 
26.2 MILLIMETRES, (Fig. 8).—At this stage the cephalic shield exhibits other features of 
maturity, such as the thickening of the test, strengthening of the posterior margin, We. 
Remains of the trilobite of this and the succeeding stage are much more common than 
those of younger individuals. k 
Anterior margin.—The marginal fold becomes a little wider in front, and at the ends 
decidely, but not greatly wider than in the 21.8 mm. size; there is also an increase in the 
width of the flat area of the anterior margin, so that the whole margin gains in width of 
sutural measurement at the expense of the ocular lobe. 
Glabella—It is in this part of the shield that the most decided change now occurs. 
There is an increase in all directions, but especially in the dome which expands laterally. 
and increases also in length, so that it begins to push out the front of the anterior margin, 
with the fold of which it is now in contact. 
Occipital ring.—This section of the shield is proportionately longer than in the younger 
stages of growth. 
The posterior margin does not change much in width, but is much shorter in propor- 
tion to the occipital ring. 
The fixed cheek is proportionately shorter, the arch of the eyelobe fuller than in the 
earlier stages. 
Sculpture.—This does not differ materially from that of the form last described. 
30.1 MILHMETRES (Fig. 7.)—This is the largest, well-preserved head which has been 
obtained, and shows a continued enlargement of the glabella. It presents some differencs 
from the younger stages which may be varietal or due to pressure and distortion. 
The anterior margin is proportionately somewhat smaller than in the 26.2 mm. size. 
Glabella—The dome is larger, both longitudinally and transversely, and the fourth 
furrow is more than half-way from the front of the dome. 
The posterior margin is somewhat longer and considerably wider than in the last, and 
the fixed cheek has gained in length and width. 
The sculpture is the same as that given in the description on a previous page of the 
species P. eteminicus. 

