ACERATA—MEROSTOMATA. 399 



rate; thoracic rings narrow beneath carpace, but attain full width 

 when exposed; pleurae scarcely distinguishable from axis. (Type 

 of genus.) 



Upper Devonic(?) (Fern Ledges) of New Brunswick. Re- 

 garded by some as Carbonic. 



Class ACERATA Kingsley. 



Subclass Merostomata Dana. 



Order XIPHOSURA Gronovius. 



Crustacea-like forms; body in mature types distinctly triolobed 

 longitudinally; cephalothorax depressed, large, semicircular; the 

 pair of compound eyes situated laterally and the pair of ocelli in 

 the center in front; six pairs of walking legs about the mouth, 

 the first pair and sometimes several succeeding pairs bearing chelae ; 

 abdomen with seven to ten segments, which dorsally may be either 

 free or united; the six anterior ones are provided with five pairs 

 of lamellar appendages on the under side, the so-called " gill- 

 books " for respiration, covered by the enlarged first pair (oper- 

 culum) ; telson long or short, sword-shaped, movable. 



Literature. 

 1885. Packard, A. S. Mem. Nat. Acad. Sci., 3 (Carbonic Xipho- 



sura of N. America). 

 1885. Williams, H. S. A. J. S. (3), 30 (Devonic). 

 1888. Hall, James, and Clarke, J. M. Pal. N. Y., 7 (Devonic). 

 1902. Rogers, Austin F. Some new American species of Cyclus 



from the Coal Measures, Kansas Univ. Sci. Bulletin I, No. 10, pp. 



269-275, pi. XIV. 



(See also. Palaeontology of Illinois. ) 



I. Cyclus deKoninck. 

 Cephalothorax small, orbicular, discoidal, or convex, calcareous 

 or chitinous, bounded by a distinct border; the imperfectly pre- 

 served appendages seem to be simple swimming legs; their en- 

 larged joints cover the ventral surface of the carapace everywhere 

 except in the center, which is occupied by a V-shaped plate, towards 

 the pointed extremity of which all the basal joints of the limbs 

 converge; Cyclus is known almost solely by its cephalothorax and 

 its poorly preserved appendages. Carbonic. 



