128 Alexander Petrimkevitch, 



protecting the genital opening. Behind this groove is another 

 transverse depression of singular form, extending across the whole 

 sternite and represented in the text figure. The meaning of this 

 structure is not at all clear. It may be part of the external genital 

 apparatus, or the cover of the organs of respiration, or it may have 

 had both functions. The following five sternites are di\aded into 

 three fields by two longitudinal lines, appearing in the specimen 

 as very low ridges. These sternites and the last or seventh are 

 clearly punctate. 



The chelicera are missing. The pedipalpi are very short, pedi- 

 form, only three last joints visible. The sternum is long and typi- 

 cally composed of three areas of which the middle one is hexagonal 

 and the other two pentagonal. The coxae are triangular, \vith 

 rounded base (apex of the triangle). The coxae of the first pair 

 of legs are contiguous throughout their entire length, produced 

 anteriorly into a sharp process. Whether this process represents 

 a maxillary lobe is very difficult to decide, since there is no evidence 

 to either prove or disprove the assumption that these coxae are 

 immovable. The coxae of the fourth pair are not quite twice as 

 long as those of the first. All coxae are smooth with exception 

 of their distal end which is punctate. Legs all preserved. Those 

 of the left side complete but for the tarsus. The fourth left leg 

 is, however, in reaUty bent under the abdomen where it lies on one 

 surface with a fern. The first right leg is quite complete, wihle 

 the other legs of the right side are broken off at the patella- tibia 

 articulation line. The trochanters are one-jointed. Patella shorter 

 than femur. The tarsus of the first leg is two-jointed, and so was 

 probably the tarsus of all legs. The legs are in order 4321. First 

 leg 14.0 mm., second (without last tarsal joint) 15.75 mm., third 

 (same way) 17.0 mm., fourth (same way) 19.25 mm. Length of 

 fourth femur 5.5 mm. 



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

 Illinois. 



Architarbus minor n. sp. 

 Plate XIII, figs. 81, 82; text figs. 81, 82. 

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

 Museum, No. 189. The dorsal and ventral surface are superimposed 

 on both the obverse and reverse of the nodule, but the structures 

 are quite clear. Total length 15.0 mm. Cephalothorax 10. o mm. 

 long in the median line, 6.6 mm. wide. Posteriorly it is produced 

 into a lobe rounded at its end and with concave sides. Anteriorly 



