CRUSTACEA. 15 



Cheeks depressed, sloping abruptly to the margins, broad and flat aboul the 

 genal angles, echinate at the extremities. 



Eyes large, scarcely reaching the height of the glabella. Visual surface 

 lunate. Corneal lenses, in the normal individual, abundant and subject to 

 considerable variation in number. Beginning at the posterior angle of the 

 visual surface and following the rows diagonally, parallel to the lower posterior 

 margin, the number of rows is generally eight, in no observed instance ex- 

 ceeding nine and rarely falling to seven. In Phacops rana the number of rows 

 is usually nine, often ten, and rarely eleven or eight. In Phacops cristata the 

 number of lenses averages about sixty, varying between the limits forty- 

 seven and eighty-two. the higher numbers being reached only in examples 

 from the Upper Helderberg limestone. Palpebrum scarcely prominent ; 

 strength of the palpebral lobe varying with the elevation of the eye. 



Thorax. Margins sub-parallel, tapering slightly backward. Surface depressed- 

 convex, and strongly trilobate. Length to width as 1 to 5. 



Axis convex and evenly tapering. 



Pleura flattened above and abruptly deflected at about one-third their 

 width from the axial furrows. The segments show a slight retral curve on 

 the axis near the margins, bending forward over the axial line: dichotomous 

 on the pleura?, the anterior limb becoming obsolete at the fulcrum by the 

 development of the lateral articulating planes. 



Pygtdium. Outline transversely semi-elliptical. Length to width as I to 2. 

 Surface convex, depressed on the pleurae. 



Axis prominent, tapering to a broad, obtuse and obscurely defined termina- 

 tion within the posterior margin ; in well-preserved specimens it bears five 

 or six annulations, and behind these three or four pairs of tubercles or unde- 

 veloped annulations, which are separated by a low axial depression. 



The pleura each bear five, and traces of six dichotomous annulations. all be- 

 coming obsolete before reaching the margin. In casts of the under surface, 

 which is the usual condition of preservation in the Schoharie grit, and often 



