aoe BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
and .80. In specimens with cephala over 30 mm. long, the growth 
seems to be more rapid in the transverse direction, and the ratio drops 
again. It is only .62 in the two largest specimens in the collection, 
the largest cephalon being 60 mm. long. The form in these large 
specimens remains subtriangular, and does not return to the semi- 
circular outline of the young. 
_Pygidia show a similar series of changes, but the index rises higher, 
and does not show so great a falling off in large specimens. In the 
adults, the pygidia are usually of about the same length, or a little 
longer, than the cephala, but are always narrower, hence the higher 
index. In specimens where the length of the cephalon is .75 the width, 
the length of the pygidium is generally .80 the width. In the larger 
specimens, where the indices of both shields have begun to drop, the 
difference is more noticeable, the index of the cephalon being .62 while 
the index of the pygidium is .77 in one well-preserved specimen. 
Raymond and Narraway found occasional specimens in which the 
length equaled the width, but in the present collection the highest _ 
index noted was .85. : 
In small specimens, the pygidium is about one half as long as the 
cephalon, and the two shields do not reach quite equal length until 
the whole animal has attained alength of about 50mm. In specimens 
more than 100 mm. long there is a tendency for the pygidium to exceed 
the cephalon in length, the difference being as much as 10 mm. in > 
some large specimens. That the pygidium does, in the adult, finally 
reach the same length as the cephalon, seems to be due, not to an 
acceleration in growth at any particular time, but rather to a retarda- 
tion of the longitudinal growth of the cephalon, during the process of 
widening. The pygidium never becomes quite as wide as the cepha- 
lon, so that more energy can be put into longitudinal growth. 
Glabella and axial lobe of the pygidium.— The glabella soon loses its 
convexity and prominence, as well as its furrows, basal lobes, and 
median tubercle. These features are still visible on a cephalon 4.5 mm. 
long, but are faint, and the front of the glabella no longer rises abruptly 
from the brim. The outline of the glabella is still distinguishable on a 
cephalon 7.5 mm. long, but it is there hardly more convex than in the — 
adult. Something of the form of the glabella can be made out on | 
most specimens, whatever the size, but it is rather smoothly merged / 
into the general surface of the cephalon in all specimens with a cepha- 
lon more than 8 mm. long. | 
The axial lobe of the pygidium remains convex and conspicuous 
somewhat longer than the glabella does. On a specimen 9.5 mm. 
