Palaeozoic Arachnida of North America. 105 



groove. A median longitudinal groove runs from this point back- 

 Ward to the posterior margin. Eyes absent. Abdomen sphaeroidal, 

 longer than v^dde, and wider than high, with the anal operculum 

 placed ventrally and surrounded by a plate composed of the fused 

 tergite and sternite of the tenth segment. Dorsal surface of ab- 

 domen covered with irregular polygonal thickenings similar to those 

 of the cephalothorax, ventral surface smooth. Two deep grooves 

 separate the tergites from the sclerites of the pleura in all segments 

 except the tenth and eleventh and probably the first. 



Ther sternum is very large, being 3.3 mm. long and 2.4 mm. 

 wide between the coxae of the second and third pair of legs. It is 

 truncated in front, has tree pairs of lateral projections and a 

 posterior bifid lobe. Legs robust and moderately long, patella 

 completely fused with tibia, tarsi longer than metatarsi. Length 

 of legs in order 4312. 



Two structures \'isible on the reverse require special mention. 

 They are : a beak-like median process directed backward, and two 

 oval deeply punctate areas. The first may be the end of the cephalo- 

 thorax turned downward. Another interpretation would be that 

 this process represents the chelicera. The punctate oval areas 

 undoubtedly are parts of the pedipalpi, perhaps a stridulatory 

 organ on the coxae. 



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

 Illinois. 



Trigonomartus woodruff i (Scudder). 

 Plate IX, fig. 52. 

 = Anthracomartus woodruffi Scudder, Bull. U. S. Geol. Surv., 

 No. loi, 1893, p. 9, pi. I, figs. 



The type and only specimen of this species is a mere fragment of the 

 dorsal surface of the abdomen. From its likeness \vith the preceding 

 species I have placed it in the same genus, since it certainly does not 

 belong to the genus Anthracomartus. The surface of the tergites is 

 considerably smoother than in T. pustidatus and the abdomen much 

 flatter. 



Found in the Pennsylvanic (? Pottsville), of Rhode Island. 



Genus Trigonotarbus Pocock 1911. 

 Cephalothorax triangular, elevated in the middle, sloping gradually 

 in all directions, not sculptured. Legs with patella. Anal operculum 

 ventral in position. Genotype T. johnsoni Pocock. 



