344 SYSTEMATIC PALEONTOLOGY 



cheeks of each are different. In 7. megistos the base of the spine is much 

 wider and it tapers much more rapidly. The hypostoma furnishes further 

 differences, for in 7. megistos it shows coarser venations, the inner edges 

 of the limb are straighter, and the whole hypostoma is relatively longer. 



Occurrence. MARTINSBURG SHALE (Fairview division, Orthorhyn- 

 chula bed). Pennsylvania to Tennessee. 



Collection. TJ. S. National Museum. 



ISOTELUS GIGAS DeKay 



Plate XLVIII, Figs. 23-25 



Isotelus gigas DeKay, 1824, Annals Lyceum Nat. Hist. New York, vol. i, 



p. 176, pi. xii, fig. 1. 



Isotelus gigas Hall, 1847, Pal. New York, vol. i, p. 231, pi. Ix-lxiii. 

 Isotelus gigas Weller, 1903, Pal. New Jersey, vol. iii, p. 192, pi. xiv, figs. 6, 7. 

 Isotelus gigas Raymond, 1914, Bull. Mus. Comp. Zool., vol. Iviii, p. 248, pi. i, 



figs. 1, 2, pi. ii, figs. 2-5; pi. iii, fig. 3. 



Description. " Outline of an entire individual subelliptical, with the 

 anterior and posterior extremities somewhat pointed; the trilobation 

 nearly obsolete. Head subtriangular to semi-elliptical in outline, convex, 

 slightly flattened in front; the anterior margin rather sharply rounded; 

 facial sutures meeting at an angle, at or just behind the frontal margin, 

 from this point they describe a broad, subarcuate curve, and after passing 

 around the eyes, they curve outward and then downward, intersecting 

 the posterior margin at some distance outside of the eyes; glabella 

 obscurely denned and more obscurely lobed; occipital furrow and seg- 

 ment obsolete: free cheeks marked by an intramarginal furrow, above 

 which their general surface is elevated into a more or less conspicuous 

 node, crowned by the eye. Thorax with a broad axial lobe, occupying 

 more than one-third the width, consisting of eight segments. Pygidium 

 subtriangular in outline of nearly the same size and shape as the head, 

 its lobation very obscure, especially in the larger individuals, the dorsal 

 furrows being hardly distinguishable ; axis much narrower at its anterior 

 extremity than the axis of the thorax, tapering rapidly to the obtusely 

 rounded posterior extremity, which lies at about one-fourth the length of 



