256 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
youthful characters usually lost by that species. Thus, the adult 
Tsotelus iowensis has long genal spines, extending to the fifth, sixth, or 
seventh segment, an axial lobe a little less than half the width of the — 
thorax (.41 to .46), and pygidia show a fairly convex axial lobe, and 
traces of ribs on the pleural lobes. The pygidia are also somewhat 
more rounded than those of Isotelus gigas, although fully as long. As 
minor features of the species, it may be noted that the anterior portion 
of the glabella is quite well defined, and the geniculation of the pleural 
lobes of the pygidium is further from the dorsal furrows than in 
Isotelus gigas. The collection contains a number of cranidia and 
pygidia, and two complete specimens from the Maquoketa at Elgin, 
Iowa, and a single complete specimen which is presumably from Iowa 
but not labeled as to locality, so that direct comparisons can be made 
with the specimens from Trenton Falls, and the specimens from the 
eastern and western localities are found to agree in all particulars. 
The specimens from Trenton Falls have usually been identified as 
Tsotelus maximus. 
Two specimens from the Lowville and Black River at Ottawa, 
Ontario, described by Raymond and Narraway (Ann. Carnegie mus., 
1910, 7, p. 56, pl. 15, fig. 3) as Isotelus sp. ind. probably belong to this 
species. They were separated from Isotelus gigas because they had 
more rounded pygidia with a rather prominent axial lobe, and they 
have a rather narrow axial lobe in the thorax. In 1910 there was no 
opportunity of comparing the specimens from Ontario with I[sotelus 
iowensis, but I now find that they agree very closely with the speci- 
mens from Iowa. 
The specimen from the “Hudson River” at Granger, Minnesota, 
figured by Dr. Clarke! as a specimen of Isotelus maximus, is, as 
figured, an excellent example of J. cowensis, the shields being too long 
and narrow for a typical specimen of I. maximus. 
The type of this species, which is now in the collection at the Walker 
Museum of the University of Chicago, was obtained from the Maquo- 
keta shale, which is of Upper Ordovician (Richmond) age, according 
to the most recent correlations. The specimens from the Trenton, 
though sometimes larger, are too much like the Iowa specimens to be 
distinguished as a distinct species at the present time. 
Additional note:— Since this paper was written Mr. A. W. Slocum’s 
excellent account (Field mus. nat. hist., publ. 171, Geol. ser. 4, no. 3, 
p. 48, pl. 13, fig. 1, 2) of the trilobites of the Maquoketa beds of Fayette 
1 Pal. Minn., 1897, 3, pt. 2, p. 703, fig. 5; also reproduced by Weller as Isotelus 
gigas, Pal. N. J., 1902, 3, pl. 14, fig. 5. 
