TRILOBITES. 721 
Bumastus trentonensis.] 
shales at St. Anthony Park and from Minneapolis have nine, and a single coiled 
example from the Galena shales at Stanton, Goodhue county, Minnesota, has but 
eight. These differences in degree of segmentation, unaccompanied by any palpable 
distinction in other respects do not afford a basis of specific separation. They are 
apparently only developmental conditions, not of the individual so much as of the 
specific type. 
Among all the western specimens examined there is no great difference in size. 
None are larger than the specimens from Trenton Falls bearing ten segments, and 
those from the Black River limestone withnine. The habit of the western specimens 
is somewhat smaller, though separated heads from Dixon, Illinois, attain the usual 
size of the New York examples. None of the specimens from Minnesota appear to 
have possessed ten segments, and this local variation is similar to that occurring 
among the Canadian examples. 
Such variations in the degree of segmentation are not, indeed, usual in the 
mature conditions of a species; they are, however, altogether in harmony with the 
laws of morphogeny, and deviations from the normal Trenton type with ten segments 
are to be interpreted as phylogenetically immature or senile phases of the specific 
type. Under the description of I//wnus milleri the detailed structure of this species 
has been clearly given by Billings. 
Fig. 34.—Front view of an enrolled individual Fig. 35.—Profile of the same specimen. 
of Bumastus trentonensis Emmons (sp.). x2. 
A peculiar feature which B. trentonensis possesses in common with B. orbicaudatus 
is a pair of lunate depressions on a transverse line between the eyes. These are 
longitudinally elongate, each about half way between the eye and the axial line, and 
much more clearly apparent on the cast than on the outer surface. It seems 
probable that such cephalic cicatrices were areas of insertion of muscular bands 
attached to similar scars on the inner surface of the hypostoma. 
Normally there is no trace of longitudinal lobation on the cephalon or pygidium, 
and the axial furrows of the thorax are very obscure. Slight vertical compression, 
however, serves occasionally to emphasize these features in the head and thorax, 
and also lessens the convexity of the former. 
Formation and locality.—Trenton limestone: Minneapolis and St. Paul, Minnesota; Dixon, Illinois; 
Platteville, Wisconsin. Galena shales: Pleasant Grove, Cannon Falls, Stanton and Kenyon, Minnesota. 
—46 
