330 Systematic Paleontology — Middle Devonian 



Phacops rana Meek and Worthen, 18G8, Geol. Surv. 111., vol. iii, p. 447, pi. xi, 



figs. la-e. 

 Phacops rana Nicholson, 1873, Pal. Prov. Ontario, p. 123, flg. 56a. 

 Phacops rana Whitfield, 1882, Geol. Wis., vol. iv, p. 339, pi. 26, figs. 17-19. 

 Phacops rana Hall and Clarke, 1888, Pal. N. Y., vol. vii, p. 19, pi. vii, figs. 



1-11; pi. viii, figs. 1-18; pi. viii A, figs. 21-33. 

 Phacops rana Keyes, 1891, Johns Hopkins Univ. Circ, vol. xi, p. 29. 

 Phacops ratia Clarke, 1903, N. Y. State Mus., Bull. 65, p. 737. 

 Phacops rana Clark and Mathews, 1906, Md. Geol. Surv., vol. vi, pi. xvii, fig. 9. 

 Phacops rana Grabau and Shimer, 1910, N. Am. Index Fossils, vol. ii, p. 323, 

 fig. 1638. 



Description. — General form elongate snboval; greatest width (measured 



at the posterior margin of tlie cephalon) to axial length as 1 to 2 ; the 



cephalon, thorax and pygidium are to one another in length as 1.5 to 3 to 



1. Cephalon subsemicircular, the regularity of the outline interrupted by 



the slight protrusion of the glabella and the genal extremities, frontal 



margin obscure, concealed by the overhanging glabella; facial sutures 



rarely discernible; glabella large, gibbous, outline subpentagonal, greatest 



width anteriorly, posterior furrow extending clear across the glabella; 



cheeks abruptly sloping to the margin, narrowing anteriorly and reflected 



ventrally to form the doublure; eyes prominent, scarce reaching the 



height of the glabella in uncompressed specimens, visual area lunate, 



separated from the cheek by a strong, smooth stilcus, average number of 



lenses in normal adults is between forty and fifty for each eye. Thorax 



subquadrate, lateral margins slowly tapering, surface strongly trilobate; 



axis flattened at the margins, evenly convex in the middle, widest at the 



third or fourth segment, tapering very slowly to the ninth, and thence 



much more rapidly to the pygidium ; pleurae flat for about one- third their 



width from the axis and thence abruptly deflected to the margin, each 



segment bears a furrow which becomes obsolete at the fulcrum. Pygidium 



relatively small, regiilarly and evenly rounded margin the posterior part 



of which forms the arc of a circle; axis composed of nine annulations, 



rapidly and evenly tapering from the last segment of the thorax, reaching 



an acute termination just within the posterior margin; pleurae seven in 



number, broad, depressed-convex, and sloping evenly to the posterior 



margin. Surface of the test ornamented with tubercles, which are largest 



and most closely set upon the glabella. 



