CRUSTACEA. 
119 
The species occurs without variation except in the occasional presence of the 
axial row of tubercles, and this peculiarity was noticed in the original descrip¬ 
tion of the species (Fifteenth Kept. N. Y. State Cab. Nat. Hist., loc. cit.), the 
variation being designated as P. macrocephalus, var. a. It appears, however, on 
closer examination, that this feature is not connected with variations in other 
respects, and individuals are found showing all intermediate stages between 
those with and those without tubercles. P. macrocephalus affords many points 
of close similarity to P. curvimarginatus, the general proportions of the two 
species and many of the details being quite identical. Differences, however, 
exist in the following respects: In the former the border is not so broad, so 
deeply excavate or so strongly reflected; the glabella is more broadly conate, 
and its lateral furrows much less conspicuous ; the genal spines are shorter and 
stouter; the pygidium more elongate and more distinctly flattened upon the 
pleurae. 
Distribution. Hamilton group. In the limestone layer of the Marcellus 
shales; Orleans, Ontario county; in the Hamilton shales of the central and 
western counties; Canandaigua Lake, at Menteth’s, Tichenor’s and Foster’s 
Points; Canandaigua, Bristol and Hopewell, Ontario county; Bellona, Yates 
county; Jaycox’s Run, near Geneseo, and Moscow, Livingston county; Bethany, 
Genesee county; Eighteen-mile Creek, Erie county; in the Tully limestone, 
below Lodi Landing, Seneca Lake, and Borodino, Onondaga county. 
Proetus Rowi. 
PLATE XXI, FIGS. 2-6, 24-26 ; AND PLATE XXIII, FIGS. 20-29. 
Calymene Rowii, Green. Amer. Journ. Science and Arts, vol. xxxiii, p. 406. 1838. 
Iroltus Roioii, Hall. Descr. New Species of Fossils, etc., p. 75. 1861. 
Proetus Rowii, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., p. 103. 1862. 
Proetus Rowii, Hall. Illusti'ations of Devonian Fossils, jil. xxi, figs. 2-6. 1876. 
General Form and Proportions. Body ovate or elongate-elliptical in outline. 
Surface depressed-convex, sub-equally trilobate. Axial length one-half 
greater than the width at the base of the cephalon. 
Cephalon semi-elliptical to semicircular in outline, width about twice the 
length. Border broad and flat; at the angles of the cheeks produced into 
