292 NORTH AMERICAN INDEX FOSSILS. 



pleura sharply turned down at the sides, pygidium without seg- 

 mentation, and surface of entire test thickly covered with large 

 punctae. The species can usually be distinguished by this last 

 character. 



Chazy of New York. 



93- !• gigas Dekay. (Asaphtis platycephalus Stokes). (Figs. 



1 60 1, 1602.) Ordovicic. 



Pygidium in adult shows scarcely any segmentation except upon 

 the internal mold. 



Black River of Canada; Trenton of North America.; also in 

 Utica of New York. 



94. I. maximus Locke. (Asaphtis megistos Locke.) (Fig. 1603.) 



Ordovicic. 



Similar to /. gigas but with genal spines. This species may 

 represent only a phase in the development of /. gigas. 



Trenton-Richmond of New Jersey, Minnesota and Wisconsin. 



XLIL Ill^nus Dalman. 



Cephalon and pygidium large, semicircular, convex, smooth ex- 

 cept for the slightly impressed dorsal furrows and the eyes. Gla- 

 bella smooth, indistinct. Facial suture describes a slightly sigmoidal 

 curve from anterior margin of cephalon to eye; from posterior 

 edge of the eye it curves rather abruptly laterally to the posterior 

 margin of the cephalon, a considerable distance in from the genal 

 angle. Free cheeks small. Eyes usually large and situated later- 

 ally. Thorax usually with ten segments and with smooth pleura. 

 Pygidium smooth, similar to the cephalon in form and size, and 

 with short and inconspicuous axis. Ordovicic and Siluric. 



A. Free cheek prolonged into genal spines I. 



I. Inner margin of free cheek with a rounded furrow 104. /. arrnaius. 



I. Inner margin of free cheek without a furrow 97. /. angusticollis. 



B. Free cheeks not prolonged into spines II. 



II. Free cheeks much prolonged laterally 95. /. consimilis. 



II. Free cheeks not prolonged laterally I. 



I. Eyes very small 103. I. imperator. 



I. Eyes large .a. 



a. Head with an upward bending of the anterior margin 11. 



II. Outline of head parabolic 102. I. insignis. 



II. Outline of head semicircular aa. 



