Palaeozoic Arachnida of North America. 123 



the other two pentagonal. Coxae of pedipalpi Wedge-shaped, con- 

 tiguous throughout their entire length. Coxae of first pair contiguous 

 at base. Coxae of fourth pair more than twice as long as those of 

 first pair. Trochanters one-jointed. Of the legs preserved only the 

 femora of the second, third and fourth leg. Pleura not segmented, 

 probably soft, appearing as heavy ridge at one side of the abdomen. 

 Body quite smooth. 



Specimen in the Museum of Comparative Zoology-, Harvard Uni- 

 versity, not numbered, but accompanied by a label which reads : 

 Libellula carbonaria Scudder, AAAS, Vol. XXIV, B, 1875. In every 

 respect similar to the type specimen, but not as Well preserved. Of 

 the cephalothorax preserved the proximal end only. Length of 

 abdomen 9.2 mm. 



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

 Illinois. 



Family Architarbidae. 

 (New definition.) Cephalothorax broadly joined to the abdomen. 

 Pedipalpi short, pediform. Coxae of first pair of legs contiguous 

 throughout their entire length. Eyes, when present, in the number 

 of two, on the cephalothorax. Anal operculum ventral in position. 

 Abdominal tergites nine to eleven, the anterior five or six much 

 shorter than the following, with a heavily thickened posterior edge, 

 typical of the order. Abdominal sternites seven or eight. Pleura 

 not segmented, soft. 



Key to the Genera of Architarbidae. 

 I. Cephalothorax produced posteriorly, with curved sides con- 

 verging to a point. Five or 6 anterior tergites distinctly pro- 

 curved. Sternum well developed 



Architarbus 

 + Cephalothorax with a straight posterior edge, broadly rounded 

 in front. Anterior abdominal tergites straight. Sternum re- 

 duced to a line 



Opiliotarbus. 



Genus Architarbus^ Scudder 1868. 

 New definition. Cephalothorax produced posteriorly or at least 

 with a very strongly procurved posterior edge. Anteriorly it is 



1 Geraphrynus is a synonym of Architarbus. See Introduction and the 

 description of Architarbus rotundatiis. 



