CRUSTACEA. L37 



Axis convex, elevated, evenly tapering to an abrupt termination distant 

 from the posterior margin; width on the anterior margin about one-third 

 the width of the shield; bearing from eight to twelve narrow annulations, 

 which have a slight retral bend along the axial line and appear to be angu- 

 lated by the five longitudinal rows of strong tubercles. Of these rows the 

 middle one is much the strongest. Upon the elevated termination of the 

 axis are a number of small, irregularly scattered pustule-. 



Pleurce gently convex for about one-half their width, and depressed upon 

 the broad, flattened border; bearing eight annulations, which are grooved 

 for their entire length, making the anterior limb of each strong and the 

 posterior limb inconspicuous. Each annulation bears upon the anterior 

 limb four, five or six strong sub-spiniform tubercles, of which those upon 

 the margin are much the strongest and are usually produced into short 

 spines. Some specimens hear a terminal spiniform tubercle upon the 

 margin between the last two annulations. The pleura) and articulating 

 grooves are punctate. 



Doublure broad and flat. 



Dimensions. A thorax from the Hamilton shales measures 6 mm. in length 

 and 10 mm. in width ; a pygidium from the same formation 6.5 mm. in length 

 and 9 mm. in width; a small pygidium from the Upper Helderberg limestone 

 3 mm. in length and 5 mm. in width. 



Distribution. Upper Helderberg group. Corniferous limestone: In the de- 

 composed chert boulders, Canandaigua. Ontario county. Hamilton group: In 

 the lower shales at Canandaigua and Fall Brook, Hopewell, Ontario county : 

 Eighteen-Mile creek, Erie county. 



Phaethonides oyceurus, n. sp. 



PLATE XXIV, FIG. 26-28; and PLATE XXV, FIG. 11. 



Cephalon. The intra-sutural portion, which is the only part known, indicates 

 a semicircular outline for the head; frontal and lateral areas depressed and 



