SEGMENTS IN TRUNK. 
131 
Only sixteen genera of Devonian trilobites were available for tabulation, and it is not 
always possible to ascertain the exact number of segments in the pygidium, although genera 
with smooth caudal shields had nearly all disappeared. The number of segments in the thorax 
had become pretty well fixed by the beginning of the Devonian, Cyphaspis with a range of 
from 10 to 17 furnishing the only notable exception. The range for the sixteen genera is 
from 8 to 17, the average 11, the number exhibited by the Phacopidae which form so large 
a part of the trilobites of the Devonian. The greater part of the species have large pygidia, 
and while the range is from 3 to 23, the average is 1 1.2. Probolium, with 11 in the 
thorax and n-13 in the pygidium, and Phacops, with 11 in the thorax and 9-12 in the 
pygidium, approach very closely to the "average" trilobite, and various species of other 
genera of the Phacopidae have the same number of segments as the norm. In every genus, 
however, the number of segments in the pygidium is variable, the greatest variation being 
in Dalmanites, with a range of from 9 to 23. The number of segments in the pygidium 
was therefore not fixed and was on the average higher than in earlier periods. 
The genera used in the tabulation were : Calymene, Diplcura, Goldius, Proetus, Cyphas- 
pis, Acidaspis, Phacops, Hausmania, Coronura, Odontochile, Pleuracanthus, Calmonia, Pen- 
naia, Dalmanites, Probolium, and Cordania. 
The trilobites of the late Palaeozoic (Mississippian to Permian) belong, with two pos- 
sible exceptions, to the Proetidoe, and only three genera, Proetus, Phillipsia, and Griffithides, 
appear to be known from all the parts. I am, however, assuming that both Brachy met opus 
and Anisopyge have 9 segments in the thorax, and so have tabulated five genera. The 
range in the number of segments in the pygidium is large, from 10 in some species of 
Proetus to 30 in Anisopyge, and the average, 17.3, is high, as is the average for total num- 
ber in the trunk, 26.3. Anisopyge, a late Permian trilobite described by Girty from Texas, 
is perhaps the last survivor of the group. It seems to have had 39 segments in the trunk, 
making it, next to the Cambrian Pcedeumias and Menomonia, the most numerously segmented 
of all the trilobites. 
The above data may be summarized in the following table : 
rt 
C 
£ 
bjD 
in 
c 
0) 
6 
M 
■S 
Period 
3 
M 

6 
2; 
O 
2 2 
> ~ 
<! .a 
Av. No. of 
in pygidiun 
Av. No. of 
segments 
Lower Cambrian 
19 
13-9 
3-7 
17.6 
Middle Cambrian 
33 
10. s 
5-9 
16.4 
Entire Cambrian 
62 
17-19 
Ordovician 
40 
10.15 
8.81 
18.96 
Devonian 
16 
11 
11. 2 
22.2 
Late Palaeozoic 
5 
9 
17-3 
26.3 
This table confirms that made up by Carpenter, and shows even more strikingly the 
progressive increase in the average number of segments in the trunk throughout the Palae- 
ozoic. 
While the two trilobites with the greatest number of segments are Cambrian, yet on the 
average, the last of the trilobites had the more numerously segmented bodies. The multi- 
segmented trilobites are: 
