136 
THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
Stage 
Length in 
mm. 
Percentage 
of pygitlium 
Segments in 
thorax 
Segments in 
pygidium 
I 
0-75 
25 

2 
2 
0-7S 
25 

3 
3 
1. 00 
25 

4 
4 
I .00 
22 
I 
3 
5 
1 -25 
20 
2 
3 
6 
I -25 
18 
3 
3 
7 
1.60 
15 
4 
3 
8 
1.60 
IS 
5 
3 
9 
3.00 
20 
6 
6 
10 
3-50 
20 
7 
6 
11 
8.00 
18 
9 
7 
12 
8.00 
16 
11 
5 
13 
12.00 
l6 
11 
7 
14 
19.00 
18 
11 
9 
15 
95.00 
20 
11 
11 
Since the above was written, Troedsson (1918, p. 57) has described the development 
of Dalmanites eucentrus, a species found in the Brachiopod shales (Upper Ordovician) of 
southern Sweden. This species follows a course similar to that of D. socialis, so that the 
full series of stages need not be described. The pygidium is, however, of especial interest, 
for there is a stage in which it shows two more segments than in the adult. Troedsson 
figures a pygidium 1.28 mm. long which has eight pairs of pleural ribs, while the adult 
has only six pairs. The ends of all these ribs are free spines, and were the development 
not known one would say that this was a case of incipient fusion, while as a matter of fact, 
it is incipient freedom. 
A further interest attaches to this case, because of the close relationship between 
D. eucentrus and D. mucronatus. The latter species appears first in the Staurocephalus 
beds which underlie the Brachiopod shales, so that in its first appearance it is somewhat the 
older. The pygidium of the adult D. mucronatus is larger than that of D. eucentrus, having 
eight pairs of pleural ribs, the same number as in the young of the latter. In short, D. eucen- 
trus is probably descended from D. mucronatus, and in its youth passes through a stage in 
which it has a large pygidium like that species. Once more it appears that the small pygidium 
is more specialized than the large one. 
The full ontogeny of Cyclopyge is not known, but young specimens show conclusively 
that segments are not transferred from the thorax to the pygidium, but that the opposite 
occurs. As shown by Barrande (1852) and corroborated by specimens in the Museum of 
Comparative Zoology, the process is as follows : The third segment of the adult of this 
species, that is, the fourth from the pygidium, bears a pair of conspicuous cavities on the 
axial portion. In a young specimen, 7 mm. long, the second segment bears these cavities, 
but as the thorax has only four segments, this segment is also the second instead of the 
fourth ahead of the pygidium. The pygidium itself, instead of being entirely smooth, as 
in the adult state, is smooth on the posterior half, but on the anterior portion has two well 
formed but still connected segments, the anterior one being more perfect than the other. 
These are evidently the two missing" segments of the thorax, and instead of being in the 
process of being incorporated in the pygidium, they are in fact about to be cast off from it 
to become free thoracic segments. In other words, the thorax grows through the degener- 
