CARBONIFEROUS ARACIIXIDA OF NORTH AMERICA. 449 



than half as long again as broad and the three succeeding joints of the same are about 

 two and a half times as long as broad, eaclamore than half as long as the cej)halothorax; 

 the second joint a little the broadest, the others tapering slightly; the joints of the fourth 

 pair ai'e not very ditFei'ent. 



The abdomen is shajaed ver}^ much as in the last species but more smoothly rounded, 

 much more elongated, being nearly twice as long as, and at its greatest only slightly 

 broader than, the cephalothorax; the lateral sulcus, only distinguishable in jilaces, is sim- 

 ilarly close to the margin. 



The whole body is coarsely, deeply and uniformly punctate, and the surface between 

 the jjunctures apparently minutely and faintly punctulate, giving a scabrous appeai-ance 

 to the whole; the same is true of the legs though the puncta would appear to be a little 

 shallower. 



Length of body, 15 mm. ; cephalothorax, 5.25 mm.; abdomen, 9.75 mm.; breadth of 

 cephalothorax, 5.25 mm. ; abdomen, 6.25 mm. ; length of second joint of third pair of legs, 

 3 mm. 



Mazon Creek, 111. Mr. R. D. Lacoe, No. 1766ab. 



Architarbus Scudder. 



Architarhus Scudd., Geol. Surv. 111., iit, 568 (1868) . 



Cephalothorax orbicular, broadly rounded in front, much smaller than the abdomen, 

 but not sej^arated from it by a marked lateral constriction. Coxae radiating from a cen- 

 tral pit. Abdomen oval, composed of nine segments, of which those on the basal half 

 are very much shorter than the others, and on the dorsal surface are forced still more 

 closely together by the lai'ge j^ostthoracic plate. 



Species of this genus are found in the coal measures of Silesia, Great Britain, and Illi- 

 nois. In addition to the one long known from the last locality, another is described 

 below. 



Architarbus rotundatum. 



ArcJutm-hus rotundatus Scudd., Geol. Surv., 111., in, 568, fig. 4 (1868). 



I have nothing to add concerning this species. It comes from Mazon Creek, 111. 



Architarbus elongatum sp. no v. 

 PI. 40, lig. 4. 



This species is i-emarkable for the great elongation of the abdomen Avhich hardly ta- 

 pers apically; it is twice as long as the cephalothorax and of equal breadth on the basal 

 half, the last segment nearly as long as broad. On one face of the opened stone there 

 appear to be only five abdominal segments, but on the opposite face, while the three 

 apical segments remain the same, the basal portion is broken by very pronounced I'idges 

 and furrows into five short segments, together not half the length of the remainder of 

 the abdomen. Only the merest fragments of the base of the legs are preserved. The 



MEMOIRS BOSTON SOC. NAT. HIST., VOL. IV. 60 



