16 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
one of the few cases where characteristics arbitrarily chosen may serve 
to define natural groups. The bumastids particularly, which are 
simple trilobites with few variable characters, seem to be susceptible 
to this sort of treatment. The chief variable characters are amount of 
convexity, ratio of length to breadth, length of dorsal furrows, pres- 
ence or absence of “lip” on cephalon and concave border on pygidium, 
and size of eyes. Almost every possible combination of these few 
characteristics seems to be present among the species, and each com- 
bination is usually exhibited by a large number of specimens. That 
these variations are not the attributes of one very plastic species but 
of a number of distinct species is shown by the geographical distribu- 
tion as well as by the numbers in each group. For example, Bumastus 
imsignis seems to be confined to a small area in the immediate vicinity 
of Chicago, and B. iozus to Joliet, Illinois and Racine, Wisconsin, 
while B. cuniculus is found in vast numbers near Milwaukee, but is 
rare in the Chicago area. Others of the species are equally local in 
distribution. 
BuMastus cunicuLus (Hall). 
Illaenus cuniculus Hall, 20th Rept. N. Y. state cab. nat. hist., 1868, p. 377, 
pl. 22, f. 12; 1870, rev. ed., p. 421, pl. 22, f. 12. Weller, Bull. Chicago 
acad. nat. hist., 1907, no. 4, pt. 2, p. 219, pl. 19, f. 1-6. 
This is by far the most common species at Wauwatosa, Wisc. 
There are several nearly complete specimens in the M. C. Z., most of 
them enrolled. They show the axial lobe of the thorax to be extremely 
wide, and the dorsal furrows very shallow. There are ten segments. 
The diagnostic specific characteristics are: — elongate, moderately con- 
vex cephalon with very narrow rim which is prominent at the front, 
but disappears before reaching the genal angles, and eyes of medium 
size. Dorsal furrows faint, hardly visible at all in front of the scar- 
like spots just inside the eyes. Pygidium elongate, moderately convex, 
with practically no depressed border. A flattening of the convexity 
near the border can be seen if the pygidium is viewed in profile. 
Measurements: — A cephalon of average size is 45 mm. long, 55 mm. 
broad; the eye 9 mm. long, or one fifth the total length. A large 
cephalon is 56 mm. long. The pygidium of an enrolled specimen 
whose cephalon is 45 mm. long is 50 mm. long and 52 mm. wide. The 
axial lobe of this specimen is 42 mm. wide and the total width of the 
thorax is 53 mm. 
Formation and locality: — Very common in the Niagaran at Wauwa- 
tosa, near Milwaukee, Wisconsin. 
