MARYLAND GEOLOGICAL SURVEY 335 



the Trenton form usually identified as Cryptolithus tesselatus Green or 

 Trinucleus concentricus Hall by the great width and decided posterior 

 recurvance of the border and by its steeper slope, the cephalon as a whole 

 being therefore more convex. C. concentricus resembles it in the last 

 respect, but its border is of less width and has fewer rows of pits. Among 

 the differences distinguishing the species from all the American species 

 of the genus is the finely punctate and not reticulate marking of the 

 glabella and lateral lobes. 



The type specimens figured were collected in the Eden shale at Coving- 

 ton, Kentucky, and in the uppermost Trenton limestone at the same place. 



Occurrence. MARTINSBURG SHALE (Eden division). Jordans Knob, 

 one and one-half miles northeast of Fort Loudon, Pennsylvania. 



Collection. U. S. National Museum. 



CRYPTOLITHUS TESSELATUS Green 



Plate LI, Figs. 19, 20 ; Plate LIT, Fig. 17 



Cryptolithus tesselatus Green, 1832, Monograph. Trilobites North America, 



p. 73, cast 38, pi. i, fig. 4. 

 Trinucleus concentricus Hall, 1847, Pal. New York, vol. i, p. 249, pi. Ixv, 



figs. 4a, c. 

 Trinucleus concentricus Weller, 1903, Geol. Surv. New Jersey, Pal., vol. iii, 



p. 192, pi. xiv, figs. 3, 4. 



Description. " Head semi-circular or subcrescent-form in outline, the 

 genal angles either destitute of spines or produced into long, slender, 

 straight spines. Glabella smooth, very prominent, ovoid in outline, the 

 widest portion being in front, with a short, blunt spine posteriorly ; cheeks 

 smooth, prominent, but depressed considerably below the glabella, from 

 which they are separated by a well-defined dorsal furrow; eyes wanting. 

 The entire anterior and lateral margins of the head are surrounded by a 

 broad, somewhat flattened or concave border, which is marked in front 

 by from three to five concentric rows of deep, rounded pits ; one or two 

 additional rows are introduced on the sides, and toward the genal angles 

 the pits often become irregularly scattered. Length, 10 mm.; width, 

 15 mm. ; convexity, 6 mm." Weller, 1903. 

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