702 THE PALEONTOLOGY OF MINNESOTA. 



[isutclus glgas maxima*. 



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 as a 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 I. gigas. Others, again, 

 notably Locke and Miller, ascribe to 7. 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, 7. 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 7. maximus as large 

 as an average 7. gigas. Among the fossils of this region 7. maximus rarely exceeds 

 a length of 60 mm., which would be small indeed for an 7. gigas, of which individuals 

 measuring 200 or 250 mm. in length are not at all uncommon. Among the Minnesota 



