342 SYSTEMATIC PALEONTOLOGY 



TKIAETHKUS BECKI Green 

 Plate LI, Figs. 17, 18; Plate LII, Figs. 18-20; Plate LVI, Figs. 7-13 



Triarthrus fcecfci Green, 1832, Monograph Trilobites North America, p. 87, 



cast 34, pi. i, fig. 6. 

 Calymene becki Hall, 1847, Pal. New York, vol. i, pp. 237, 250, pi. Ixvi, 



figs. 2a-k. 



Description. " General form an elongated ellipse, with the posterior 

 extremity narrower, and the sides often straight ; buckler broadly semioval, 

 the posterior angles rounded ; glabella of equal width from base to front, 

 rounded before, deeply trilobate on each side, with a prominent thoracic 

 ring at the base; frontal lobe narrowed longitudinally; thorax with 13 

 segments, those of the central lobe with a short spine or tubercle upon .the 

 back, those of the lateral lobes deeply grooved along the center; caudal 

 shield with six or seven segments in the middle lobe, and five in the lateral 

 lobes; posterior extremity obtuse." Hall, 1847. 



Specimens of Triarthrus occur at several horizons in the Martinsburg 

 shale of Maryland, and although their preservation is not always good, all 

 seem to have the tubercle on the center of each axial segment, character- 

 istic of the well-known and widely distributed T. becJci Green. 



Occurrence. MARTINSBUEG SHALE. Chambersburg, Pennsylvania 

 (Sinuites bed). Williamsport, Maryland (Corynoides bed). Fort 

 Loudon, Pennsylvania, and Eickard Mountain, Maryland (Eden 

 division). 



Collections. Maryland Geological Survey, II. S. National Museum. 



Family ASAPHIDAE 



Genus ISOTELUS DeKay 



ISOTELUS STEGOPS Green 



Plate LVI, Figs. 3, 4 



Isotelus stegops Green, 1832, Monograph Trilobites North Amer., p. 71, 

 cast 26, 27. 



Description. The species of Isotelus occurring in the Eden shale of the 

 Ohio Valley hitherto referred to Isotelus gigas DeKay seems to represent 

 a distinct species for which the name Isotelus stegops Green is here 



