16 PALEONTOLOGY OF NEW YORK. 



in the Upper Helderberg limestone, the bifurcate character of the pleural 

 annulations is notalways readily discernible. 



Htpostoma elongate-triangular. Length to width as 1 to 1.25. Antero-lateral 

 angles strongly auriculate. Anterior margin with a gentle forward curve, 

 lateral margins tapering evenly to the posterior extremity, which bears a short 

 medial spine with a smaller spine on either side. Centrum convex; posterior 

 sulcus conspicuous; postero-lateral depressions faint; a pair of circular 

 depressions near the anterior angles. 



Surface Ornamentation. The surface of the glabella is covered with low, 

 closely set tubercles, traces of which seldom appear in the casts of the 

 under surface. The other portions of the shield are smooth, except 

 the genal angles which are faintly pustulose, each bearing a short, stout 

 spine. At the center of the occipital ring is a similar, though consider- 

 ably stronger spine, the first of a row of spines, twelve in number, ex- 

 tending along the axial line to the pygidium. In young individuals these 

 spines are low and inconspicuous, and are usually accompanied by a few faint 

 tuberculations upon the axis. The surface of the pygidium is nearly or quite 

 tree of ornament in the adult, and faintly pustulose in the young individual, 

 the pustules being coarser upon the axis and finer upon the pleurae, where 

 they are arranged in single rows upon each ridge of the annulations. 



Dimensions. Entire individuals referable to the type form of this species, are 

 very rarely found, although fragments are of frequent occurrence. A single 

 nearly entire example from the Schoharie grit has the following dimensions: 



Body. Cephalon. Thorax. Pygidium. 



Length 47 mm. 14 mm. 23 mm. 10 mm. 



Width 26 mm. 26 mm. 20 mm. 20 mm. 



An entire cast of the lower surface from the Corniferous limestone gives the 



following measurements : 



Body. Cephalon. Thorax. Pygidium. 



Length 75 mm. 19 mm. 43 mm. 13 mm. 



Width 38 mm. 38 mm. 35 mm. 24 mm. 



These dimensions may be regarded as those of normal full growth. 



