132 
THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
Period . 
Genus 
en 
c 
4) 
a 
bo 
™ 5 
if) 
B S 
in 
G 
0) 
a 
a) 
o o 
6 ^ 
55 .S 
o g 
o 
Z .5 
6 
Lower Cambrian 
Padeumias 
44+ 
I 
45+ 
Upper Cambrian 
Menomonia 
42 
4 
46 
Bctenonotus 
12 
22 
34 
Ordovician 
Encrinurus 
II 
22 
33 
Dionide 
6 
26 
32 
Silurian 
Harpes 
29 
3 
32 
Devonian 
Coronura 
II 
23 
34 
Dalmanites 
II 
23 
34 
Permian 
Anisopyge 
7+(9?) 
30 
39? 
Anisopyge, the last of the trilobites, stands third on the list of those having great 
numbers of segments, and in each period there are a few which have considerably more 
than the average number. It may be of some significance that of these nine genera only 
Pcedeumias and Anisopyge belong to the Opisthoparia, the great central group, and that five 
are members of the Proparia, the latest and most specialized order. 
FORM OF THE SIMPLEST PROTASPIS. 
It would naturally be expected that the young of the Cambrian trilobites should be 
more primitive than the young of species from later formations, and Beecher (1895 C) 
has shown that this is the case. He had reference, however, chiefly to the eyes, free cheeks, 
and spines, and by comparison of ontogeny and phylogeny, demonstrated the greater sim- 
plicity of the protaspis which lacked these organs. It remains to inquire which among the 
other characteristics are most fundamental. 
Among the trilobites of the Lower Cambrian, no very young have been seen except 
of Mesonacida 3 .. Of these, the ontogeny of Elliptocephala asaphoides Emmons is best known, 
thanks to Ford, Walcott, and Beecher, but, as the last-named has pointed out, the actual 
protaspis or earliest shield has not yet been found. The youngest specimen is the one roughly 
figured by Beecher (1895 'C, p. 175, fig. 6). It lacks the pygidium, but if completed by 
a line which is the counterpart of the outline of the cephalon, it would have been 0.766 mm. 
long. The pygidium would have been 0.183 mm - lon g» or 2 3 P er cent of tlie whole length. 
The axial lobe was narrow, of uniform width along the cephalon, showed a neck-ring and 
four indistinct annulations, but did not reach quite to the anterior end, there being a mar- 
gin in front of the glabella about 0.1 mm. wide. The greatest width of the cephalon was 
0.66 mm., and of the glabella 0.233 mm., or practically 35 per cent of the total width. 
Other young Elliptocephala up to a length of 1 mm., and young Pcedeumias, Mesonacis, and 
Holmia (see Kiter, Videnskaps, Skrifter, I Mat.-Naturv. Klasse, 1917, No. 10) show about 
the same characteristics, but all these have large compound eyes on the dorsal surface and 
specimens in still younger stages are expected. It may be pointed out, however, that in 
these specimens the pygidium is proportionately larger than in the adult. Walcott cites one 
adult 126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5 per cent of 
the total length, while in the incomplete specimen described above, it was apparently 23 per 
