I30 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
genera, and tends to indicate that it must be somewhere near the real average. If the 5 
or 6 segments of the head be added, it appears that the "average" number of segments is 
very close to the malacostracan number 21. Genera with 16 to 18 trunk segments are Cal- 
lavia, Protypus, Bathynotus, Crepicephalus, Bathyuriscus, Ogygopsis, Burlingia, Orria, Asa- 
phiscus, Blainia, Zacanthoides, Neolenus, Anomocare, Conocoryphe, Saukia, Olenus, and 
Eurycare. 
The order Proparia originated in the Cambrian, and Walcott has described four genera, 
one from the Middle, and three from the Upper. The number of segments in these genera 
is of interest. Burlingia, the oldest, has 14 segments in the thorax and 1 in the pygidium. 
Of the three genera in the Upper Cambrian, Norwoodia has 8-9 segments in the thorax and 
3-4 in the pygidium; Millardia 23 in thorax and 3-4 in pygidium; and Menomonia 42 in thorax 
and 3-4 in pygidium. It is of considerable interest and importance to note that the very elon- 
gate ones are not from the Middle but from the Upper Cambrian. 
Forty genera of Ordovician trilobites known from entire specimens were tabulated, and 
it was found that the range in the number of segments in the thorax and pygidium was 
surprisingly large. Agnostus, which was not included in the table, has the fewest, and 
Eoharpes, with 29, the most. While the range in number of segments in the thorax is 
2 to 29, the range of the number in the pygidium, 2 to 26, is almost as great. A species 
of Dionide has 26 in the pygidium, while Remopleurides and Glaphurus have evidence of 
only 2. The average number of segments in the thorax for the forty genera was 10.15, m 
the pygidium 8.81, and the average number for the trunk 19. 
Genera with just 19 segments in the trunk appear to be rare in the Ordovician, a 
species of Ampyx being the only one I have happened to notice. Calymene, Tretaspis, Triar- 
thrus, Asaphus, Ogygites, and Goldius come with the range of 18 to 20. Goldius, with 
10 segments in the thorax and (apparently) 8 in the pygidium, comes nearest to the averages 
for these two parts of the trunk. Goldius, Amphilichas, Bumastus, Acidaspis, Actinopcltis, 
and Sphccrexochus are among the genera having 10 segments in the thorax, and there are 
many genera which have only one or two segments more or less than 10. 
In most Ordovician genera, thirty-five out of the forty tabulated, the number of seg- 
ments in the thorax is fixed, and the variation is in any case small. In four of the five 
genera where it was not fixed, there was a variation of only one segment, and the greatest 
variation was in Pliomerops, where the number is from 15 to 19. This of course indicates 
that the number of segments in the thorax tends to become fixed in Ordovician time. The 
variation in the number of segments in the pygidium is, however, considerable. It is 
difficult in many cases to tell how many segments are actually present in this shield, as it 
is more or less smooth in a considerable number of genera. Extreme cases of variation 
within a genus are found in Encrinurus, species of which have from 7 to 22 segments in 
the pygidium, Cybcloides with 10 to 20, and Dionide with 10 to 26. As the number in the 
thorax became settled, the number in the pygidium became more unstable, so that not 
even in the Ordovician can the total number of segments in the trunk be said to show any 
tendency to become fixed. 
The genera used in this tabulation were : Eoharpes, Cryptolithus, Tretaspis, Trinucleus, 
Dionide, Raphiophorus, Ampyx, Endymionia, Anisonotus, Triarthrus, Remopleurides, Bath- 
yurus, Bathyurellus, Ogygiocaris, Asaphus, Ogygites, Isotelus, Goldius, Cyclopyge, Amphili- 
chas, Odontopleura, Acidaspis, Glaphurus, Encrinurus, Cybele, Cybeloides, Ectenonotus, 
Calymene, Ceraurus, Pliomera, Pliomerops, Pterygometopus, Chasmops, Eccoptochile, Acti- 
nopcltis, Sphcerexochus, Placoparia, Pilekia, Selenopeltis, and Calocalymene. 
