Palaeozoic Arachnid a of North America. 115 



From the Pennsylvanic (Lower Allegheny) of Mazon Creek, 

 Illinois. 



Family Heterotarbidae. 

 Hcierotarbus n. gen. 

 I. H. ovatus n. sp. 



From the Pennsylvanic (Lower Allegheny) of Mazon Creek, 

 Illinois. 



DESCRIPTION OF NORTH AMERICAN SPECIES OF 



PHALANGIOTARBI. 



Heterotarbidae n. family. 



Coxae of first pair of legs separated by the coxae of the pedipalpi. 



Coxae of the third and fourth pair of legs elongated meeting in the 



median line. First pair of legs long and slender, the other legs short 



and stout. Chelicera small, chelate. Pedipalpi not known, their 



trochanter large. Cephalothorax triangular. Abdominal tergites 



ten, the first five much shorter and typical of the order. 



Heterotarbus n. gen. 

 With the characters of the family. Genotype H. ovatus n. sp. 



Heterotarbus ovatus n. sp. 

 Plate X, fig. 59 ; text fig. 70. 



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

 collection of Mr. L. E. Daniels. The dorsal and ventral surface are 

 superimposed both on the obverse and reverse of the nodule. The 

 demarcation lines between the sternites being considerably fainter 

 than those between the tergites, especially in the anterior region of 

 the abdomen where the narrow tergites have a thickened poster or 

 edge, the sternites cannot be counted safely. The first sternite is the 

 only one clearly visible and is large and distinctly triangular. For the 

 same reason the posterior edge of the cephalothorax is not clearly 

 defined. 



Total length probably 14.0 mm., but the end of the abdomen is 

 broken off, so that the actual length cannot be given. Cephalothorax 

 triangular, 7.3 mm. wide in the region of the first tergite ; its probable 

 length 4.8 mm. The anterior five abdominal tergites very short, 

 the first and second slightly procurved, the third with a straight pos- 

 terior edge, the fourth and fifth slightly recurved. Each of these ter- 

 gites has a median ridge. The following tergites are much longer. 



