260 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
than 20 mm. long are in the matter of size and the shortening and 
thickening of the genal spines. 
Nearly all the specimens in the Dyer collection are either incomplete 
or enrolled, so that the specimens have to be considered, not in terms 
of their total length, but in accordance with the length of one of their 
shields. The smallest specimen in the collection has a pygidium 
6 mm. long, and the largest pygidium is 128 mm. long. This species 
differs from I. gigas in that the two shields become equal at an early 
age, and stay equal throughout the greater part of the adult stage. 
One adult specimen, (cephalon 46 mm. long), has the pygidium shorter 
and more nearly semicircular than the cephalon, which is the reverse 
of what is found in J. gigas. There are not enough good specimens to 
show whether this is the general rule in this species. According to 
the measurements of thirteen specimens from Cincinnati, the length of 
the cephalon averages a trifle less than .6 the width, and the pygidium 
is usually a little longer, the average being .64. The axial lobe of the 
thorax averages about .42 of the total width, and there is surprisingly 
little variation, the extremes being .40 and .44._ The axial lobe is then 
generally a little narrower than in I. gigas. 
Isotelus maximus from Toronto, Ontario.— Through the courtesy of 
Prof. W. A. Parks, I have been able to study a series of very fine speci- 
mens in the collection at the University of Toronto. The specimens 
were all from the Lorraine in the vicinity of Toronto, and were very 
well preserved, though generally a little flattened. So far as the | 
writer could determine from the large collection at the University, 
and from a short experience in the field, [sotelus maximus is the only — 
asaphid present in the Lorraine at Toronto. The specimens are quite 
large, ranging from 70 to 285 mm. in length, and they are extended, | 
not enrolled as is generally the case with the specimens from Cincin- 
nati. They show remarkably short, wide cephalons, the average — 
ratio of length to width of 11 cephalons, 9 of which were over 50 mm. 
long, being .46, and the range of variation, .43 to .51. The pygidia | 
are in all cases longer than the cephalons, and the ratio for fifteen 
specimens averages .59. The axial lobe of the thorax also averages a 
little narrower than in specimens from Cincinnati, the average of 
fifteen specimens being .374, and the limits of variation .34 to .40. 
The largest specimen which was well enough preserved to yield — 
accurate measurements was 262 mm. long. The cephalon made up | 
.30 of the length, the thorax .33, and the pygidium .37. The greatest 
width was .69 of the length. The Dyer collection contains a poorly — 
preserved specimen from Morrow, Ohio, seven of whose thoraei¢ 
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