702 THE PALEONTOLOGY OF MINNESOTA. 
[Isotelus gigas—maximus. 
individual, both show that the cephalon was without cheek spines. In Locke’s 
figures (in part a restoration) of his I. maximus published in 1841, the cheek spines 
are given full importance asa differential character. Writers have found apparently 
distinctive differences in some other respects; a broader, more obtusely angular 
head and tail-shield and a relatively wider thoracic axis in J. gigas. Others, again, 
notably Locke and Miller, ascribe to [. maximus the broader, more crescentic 
shields. 
My observations upon extensive series of these two forms from the New York 
Trenton have convinced me that specimens of each, preserved without casual distor- 
tion of the parts furnish positively no basis for a specific distinction in any of these 
respects, while it is easy to find grades of difference in these features varying with the 
degree of vertical compression of the test. Normally, in both the spinous and 
aspinous forms, the cephalon and pygidium are elongate subtriangular, the extremities 
being subacute, slightly flattened or extenuate. The facial sutures meet at an acute 
angle at, or just behind the frontal margin. The glabella is obscurely defined and more 
obscurely lobate, traces only of the lateral furrows*being visible in an oblique light. 
The cheeks bear an intramarginal furrow, above which their general surface is 
elevated into a more or less conspicuous node, crowned by the eye. The occipital 
ring and furrow are quite obsolete. The axial furrows of the thorax are distinct, the 
axis itself broad, considerably more than one-third the width of the thorax. The 
lobation of the pygidium is very obscure. The dorsal furrows being hardly distin- 
guishable, The axis is much narrower at its beginning on the pygidium than at its 
termination on the thorax, but in mature specimens its outline is scarcely discernible. 
Even a slight compression of the test, bringing the thinner or less resistant portions 
under strain gives an unnatural distinctness to the lobation of the cephalon and 
pygidium and likewise an abnormal width to the axis. The specimens of both of 
these forms from the schistose strata of Minnesota and Ohio more generally evince 
these effects of compression than those from the purer and more homogeneous 
limestone of Trenton Falls. 
The specific type of these forms being in general the same, there are still to be 
considered the important points of difference at first mentioned. It is, in a general 
sense, true of the New York examples that the aspinous head shield occurs only 
in individuals of large size; that is to say, I. gigas is almost invariably a large asaph. 
I have not seen a well defined and clearly indubitable specimen of the aspinous 
head as small as the average spinous cephalon, nor a head of J. maximus as large 
as an average I. gigas. Among the fossils of this region J. maximus rarely exceeds 
a length of 60 mm., which would be small indeed for an L. gigas, of which individuals 
measuring 200 or 250 mm. in length are not at alluncommon. Among the Minnesota 
