104 
PALEONTOLOGY OF NEW YORK. 
ginal pair probably connected with the natatory appendages, the axial pair 
possibly attached to the branchial apparatus, or to the viscera. The 
function of the median pits upon alternate grooves is not understood. Similar 
though not so distinctly preserved markings have been noticed upon pygidia of 
Proetus crassimarginatus, and the paired marginal impressions are well exhibited 
in a pygidium of Phacops cristata, var. pipa (see description of this variety, page 
18). Since the existence of a ventral membrane in certain species of trilobites 
has been demonstrated by Walcott, it may be assumed that the same structure 
has existed in this species also, and that the muscles, which are represented by 
these impressions, lay within this membrane. The specimens showing these 
features give no evidence of similar impressions upon the axial grooves of the 
thorax. 
Distribution. Upper Helderberg group. Corniferous limestone; LeEoy, 
Genesee county; Schoharie, Schoharie county; Marbletown, Ulster county. 
In boulders of decomposed chert: Canandaigua, Ontario county; Ann Arbor. 
Michigan. 
Proetus clarus. 
PI.ATE XX, FIGS. 12-14; and PLATE XXII, FIGS. 28-30. 
Proiitus clams, Hali,. Descr. New Species of Fossils, etc., p. 71. 1861. 
Proetus clams. Hall. Fifteenth Kept. N. Y. State Cab. Nat. Hist., p. 99. 1862. 
Proetus clams. Hall. Illustrations of Devonian Fossils, pi. xx, figs. 12-14. 1876. 
General Form and Proportions. Outline elliptical or sub-ovate. 
Surface convex, conspicuously and equally trilobate. Length to width as 
7 to 5. 
Cephalon. Outline semi-elliptical, hiintly produced on the frontal margin. 
Surface, in normally preserved specimens, very convex, depressed about the 
border, which is broad, rounded or sloping, thickened on the posterior por¬ 
tion, and generally bearing a broad and shallow groove in front; produced 
at the genal angles into short, acute spines, which reach to the fourth thoracic 
segment. The marginal sulcus, within the border, is deep and conspicuous. 
