CRUSTACEA. 103 



Ptoidium 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. 



Pleura 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 nun. 



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 Lelloy, 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- 



