CRUSTACEA. 
103 
Pygidium sub-semicircular, border broad, thickened and rounded. Surface con¬ 
vex, sloping more abruptly at the sides than posteriorly. 
Axis having less than one-third the width of the shield, tapering to a broad 
and blunt termination considerably within the posterior margin. It bears 
seven or eight annulations, which bend forward for a short distance within 
the axial furrows, are there sharply angulated and cross the axial line in a 
broad curve. 
Pleum with seven or eight annulations, each of which is broad, low, often 
indistinct, and very faintly grooved, all becoming obsolete upon the border. 
Length to width as 1 to 1.8. 
Dimensions. An average adult affords the following measurements; 
Body. Cephalon. Thorax. Pygidium. 
Length_ 43 mm. 12 mm. 18 mm. 13 mm. 
Width_ 27 mm. 27 mm. 23 mm. 20 mm. 
The smallest individual observed measures 24 mm. in length and 14 mm. 
in width. 
Observations. This very characteristic and well-defined species is not of com¬ 
mon occurrence but is well represented by two entire individuals from the Cornif- 
erous limestone near LeRoy, Genesee county. One of these specimens, from 
which the crust of the pygidium has been broken away, shows, upon the very fine¬ 
grained, compact matrix beneath the axis, a series of paired impressions which, 
as preserved in the cast, appear as elevations. Each transverse axial groove 
near the axial margins, bears a pair of narrow elongate impressions, which 
take their origin upon the edge of the annulation and are directed backward 
and inward into the axial groove. Within this pair and upon the upper sur¬ 
face of the axial groove is a pair of obliquely transverse, lamellate impressions, 
separated by a faint thickening of each annulation upon the axial line. Upon 
the second, fourth and sixth axial groove, along the axial line, is a faint 
tubercle, indicating a circular pit or depression in the under surface of the test. 
The upper surface of the crust shows no indications of these characters. These 
paired impressions seem to be areas of insertion for somitic muscles, the mar- 
