Palaeozoic Arachnida of North America. 49 



2.2 mm. wide at anterior edge, which is straight like the posterior 

 and the side edges. All angles rounded. A median ridge runs 

 in the specimen from the posterior edge forward, soon dividing 

 and forming two curved ridges surrounding the eye tubercle and 

 ending in the anterior corners. Median eyes small, oval, about 

 ^/s the length of the cephalothorax from the anterior edge. No 

 side eyes visible. The tergites of the abdomen gradually increasing 

 in length, the seventh being the longest. The anterior edge of the 

 fourth, fifth and sixth tergite has a sharp projection in the middle. 



The sternum is very large, distinctly pentagonal, with a recurved 

 posterior edge. Apparently both the first and second coxae have 

 maxillary lobes and the second pair is with its posterior edge abutting 

 against the sternum, so that the third and fourth pair are abutting 

 only against the side edges of the sternum, a condition completely 

 resembling the arrangement of coxae in recent scorpions, such as 

 Opisthacanthus. The genital opercula occupy the whole width of 

 the sternum and have the shape represented in text figure ii. 



Of the post-abdomen or cauda is left only the last segment with 

 the poison gland, both impressed on the seventh abdominal tergite. 

 The tail was evidently bent sidewise and over the tergite when 

 the dead specimen was imbedded in the mud. From the size of the 

 last segment as well as from comparison with the cauda of P. mazo- 

 nensis it is evident that the tail was very small and slim and it seems 

 strange that the seventh tergite has such a wide posterior edge. 

 We must conclude that the cauda was connected with it by means 

 of a comparatively large soft membrane, a condition unlike anything 

 known in modern scorpions. 



Both mandibles are preserved except for their basal joint and 

 are almost egg-shaped. 



The palpi are characterized by a short patella and a hand with, 

 very long fingers, characteristic of the family. Femur 2.35 mm, 

 patella 1.75 mm., tibia with finger 6.65 mm. The length of the 

 movable finger is 5.0 mm. 



Of the legs are left only the trochanters and femora of the second 

 and third pair and apparently a tibia of the fourth pair. 



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

 IlHnois. 



Palaeopisthacanthus mazonensis n. sp. 



Plate II, fig. I ; text figs. 13, 14. 



The type and only specimen of this species is in the collection 



of the U. S. National Museum under the number 37977. The 



Trans. Conn. Acad., Vol. XVIII. 4 June, 1913. 



