136 PALEONTOLOGY OF NEW YORK. 



Dimensions. An average cephalon measures 6 mm. in length and 6 mm. 

 in width; the pygidium 8 mm. in length and 10 mm. in width. 



Observations. In the outline and contour of the cephalon, Phaithonides vari- 

 cella approaches Cyphaspis ornata, of the Hamilton group. Its more elevated 

 and sharply recurved frontal margin, and the absence of the peculiar marginal 

 ornamentation of that species, will serve as distinguishing characters of the 

 head. The species is thus far represented by three specimens, two imperfect 

 cephala and a single pygidium. Of these, one head and the pygidium occur in 

 juxtaposition, so that little doubt remains of their specific identity. 



Distribution. Upper Helderberg group. Corniferous limestone: In the 

 boulders of decomposed chert, Canandaigua, Ontario county. 



PHAETHONIDES GEMM2EUS, 11. Sp. 

 PLATE XXIV, FIGS. 32-36. 



Cephalon unknown. Two detached thoraces have been found, one in close 

 association with a pygidium, and each of them shows seven segments. It is, 

 however, probable that some of the segments have been lost. 



Thorax sub-quadrate, margins nearly parallel. 



Axis strongly arched and very broad, being nearly one-half the width of 

 the body on the anterior margin; width increasing backward for a short 

 distance and thence tapering rapidly toward the pygidium. Each segment 

 is very narrow, elevated upon the anterior margin, grooved and depressed 

 behind. 



Pleura convex for about one-third their width, thence abruptly depressed 

 to the margins ; sulcate and strongly beveled for two-thirds their length. 



The axis bears from three to five longitudinal rows of small pustules, one 

 ol which is in the axial line, and the pleural bear each three or four similar 

 rows. The articulating grooves of the axis are punctate. 



Pygidium. Outline sub-semicircular, anterior margin curved backward at the 

 anterolateral angles. Surface sharply elevated along the median line, de- 

 pressed-convex on the lateral portions. 



