i68 NORTH AMERICAN INDEX FOSSILS. 



CXXIIL Haploceras Zittel. 



Rather involute ammonites with narrow umbilicus ; rounded 

 venter ; smooth or with sides marked by falcate growth lines which 

 outHne the ventral and lateral lappets; suture finely divided with 

 two to four auxiliary lobes; and deeply incised saddles; long, 

 straight, and bifid antisiphonal lobe, and blunt siphonal saddles. 

 Jurassic-Cretacic. 



277. H. fialar (Oppel). Jurassic. 

 Narrow, with nearly flat sides, rather large umbilicus for the 



genus, sides marked by a revolving groove slightly nearer the 

 umbilical side of the center, and by faint falcate ribs above this 

 groove. 



Kimeridgean of Mexico and Europe. 



278. H. transatlanticum Burckhardt. Jurassic. 

 Somewhat more rapidly widening than preceding, umbilicus 



smaller shell more involute ; revolving groove absent, but in its 



Fig. 1416. a, Haploceras zacatecanum, suture ; b, Eurynoticeras zitteli, suture. 

 (After Burckhardt.) 



place the growth lines make a sudden recurvation; no ribs. 

 Kimeridgean of Santa Rosa, Mexico. 



279. H. zacatecanum Burckhardt. (Fig. 1416,0.) Jurassic. 

 Differs from preceding in its somewhat narrower umbilicus and 



ribs of same strength as in H. fialar, but no groove ; umbilical 

 border subangular. 



Occurs with the preceding. 



280. H. ordoiiezi Aguilera. Jurassic. 

 Larger than preceding with broadly rounded venter, abrupt but 



rounded umbilical shoulder and numerous narrow but distinct 

 falcate ribs; no groove. 



Kimeridgean of Sierra de la Zuluaga, Mexico. 



281. H. costatum Burckhardt. Jurassic. 

 Ornamentation pronounced, the costai stronger than is usual 



