Palaeozoic Arachnida of North America. 131 



but form an angle with the strongly procurved posterior edge. The 

 sides of the cephalothorax converge anteriorly to a sharp point, 

 forming an almost spine-like projection. The whole surface of the 

 cephalothorax is smooth. Eyes apparently absent. Six anterior 

 tergites With heavily thickened posterior edge. Behind the sixth 

 tergite are visible only two lines, but it seems probable that the 

 abdomen had ten tergites and that the line separating the ninth 

 from the tenth tergite became obhterated. Anal operculum not 

 visible. Sternum badly deformed but apparently typical. Pedipalpi 

 small, pediform, only the terminal three joints visible. Coxae of 

 the first pair of legs contiguous throughout their entire length, with 

 an internal distal lobe. Trochanters one-jointed. The trochanter 

 of the second leg appears to be two-jointed, but the distal joint 

 represents apparently the soft membrane connecting the trochanter 

 with the femur and distended because the leg lies on its dorsal 

 surface. Tarsus of first leg two-jointed. Patella longer than femur. 

 Found in the Pennsylvanic (Lower Allegheny) of Mazon Creek, 

 Illinois. 



Genus Opilioiarbus Pocock 1900. 

 New definition. Cephalothorax with a straight posterior edge, 

 perfectly rounded anteriorly. Anterior six tergites very short, 

 straight, with a heavily thickened posterior edge. Number of ter- 

 gites eleven. Sternum very narrow, almost reduced to a narrow 

 ridge. Number of sternites seven. Anal operculum ventral. Coxae 

 of the first pair of legs contiguous throughout their entire length, 

 without internal apical process. Trochanter of the first and second 

 pair of legs one-jointed, of the third and fourth pair two-jointed. 

 Eyes absent. Genotype 0. elongatus (Scudder). 



Opilioiarbus elongatus (Scudder). 

 Plate XI, figs. 62-65; text figs. 85-88. 



= Architarhus elongatus Scudder, Mem. Boston Soc. Nat. Hist., 

 Vol. IV, 1890, p. 449, pi. 40, fig. 4. 



0. elongatus Pocock, Geol. Mag., (V), Vol. VII, 1910, p. 511. 



Two specimens in the collection of the U. S. National Museum. 

 Specimen No. 37975 from Mazon Creek is not as well preserved 

 as the type specimen ; but the end of the abdomen is not broken 

 off and the details of the structure are sufficiently clear. The 

 proportions of the body are somewhat different from the type, but 

 not sufficient, in my mind, to constitute a different species. The 

 text figures being carefully drawn to scale reveal no tangible 



