CRUSTACEA. Ill) 



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., he. tit.), 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 reilected ; 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. 'j 6,21 26; and plate xxiii, figs. 20-29. 



Calyniene Rowii, Green. Amer. Jouvn. Science and Ails, vol. xxxiii. p. 406. 1838. 



Proetus Bowii, Hall. Descr. New S] ies of Fossils, etc., p. 75. 1861. 



Proetus RovAi, Hall. Fifteenth Rept. N. Y. State Cab. Nat. Hist., p. Id::. 1862. 

 Proetus Sxnmi, Hall. Illustrations of Devonian Fossils, pi. 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 Hat; at the angles of the cheeks produced into 



