6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
broad, its width equaling or exceeding one half the total width of the 
body, to the genus Bumastus. This practice will probably prevail, 
for we now know that other illaenids beside Bumastus have ten seg- 
ments in the thorax, and there is no illaenid known from which dorsal 
furrows are absolutely absent. In view of the somewhat numerous 
subdivisions of the illaenids, it may be well to reéxamine the type- 
species, Bumastus barriensis Murchison, in a little detail. Salter has 
explained that the specimens figured by Murchison really do not 
belong to this species, so that we are obliged to use Salter’s figures of 
the “Barr Trilobite.” Fortunately the M. C. Z. collection contains 
two plaster casts of the original specimen figured by Jukes in 1829 
and later by Salter. The casts are rather carelessly made, but are in 
general in fair agreement with Salter’s figures. From these sources 
may be derived the statement that the typical species of Bumastus 
is a large Silurian illaenid with smooth, subequal cephalon and pygi- 
dium, rounded, spineless genal angles, large eyes, situated near the 
posterior margin of the cephalon, a very wide axial lobe, shallow dorsal 
furrows, ten segments in the thorax, and no trace of an axial lobe on 
the pygidium. The dorsal furrows on the cephalon are short, extend- 
ing but little ahead of the eyes. The cephalon does not appear to 
have any rim or concave depression, but the pygidium shows a slight 
concavity, so that the profile of that member does not present a 
smooth convex curve, but the curvature is reversed near the posterior 
end of the pygidium. Both cephalon and pygidium are wider than 
long. 
ARCHEGONUS Burmeister, 1843. 
Die organisation der trilobiten, 1848, p. 120, 121, pl. 5, f. 3. 
The type of this genus is Calymene ? aequalis H. von Meyer, as this 
was the only species cited by Burmeister in the first edition of his 
Organisation der trilobiten. As the type is evidently not an illaenid, 
but one of the Proetidae, the genus automatically disappears from the 
family. 
DyspLanus Burmeister, 1843. 
Die organisation der trilobiten, 1848, p. 120. 
Type, ILLAENUS CENTROTUS Dalman. 
The type of this genus is an illaenid with rather long, parabolic 
head and abdomen-shields, spines at the genal angles, small eyes far 
