104 PALAEONTOLOGY 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 

 Vroetus crassimarginatus, and the paired marginal impressions are well exhibited 

 in a pygidium of Pkacops cristata, var. pi-pa (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 : LeRoy, 

 Genesee county ; Schoharie, Schoharie county ; Marbletown, Ulster county, 

 In boulders of decomposed chert: Canandaigua, Ontario county; Ann Arbor. 

 Michigan. 



Proetus claeus. 



PLATE XX, FIGS. 12-14 ; and PLATE XXII, FIGS. 28-30. 



/', bus clarus, Hall. Descr. New Species of Fossils, etc., p. 71. 18G1. 

 Proetus clants, B ill. Fifteenth Rept. N. Y. State Cab. Nat. Hist., p. 99. 1862. 

 Proetus clarus, Hall. Hlustrations 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, faintly produced on the frontal margin. 

 Surface, in normally preserved specimens, very convex, depressed about the 

 border, which is broad, roundel 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 

 The marginal sulcus, within the border, is deep and conspicuous. 



