1 88 NORTH AMERICAN INDEX FOSSILS. 



320. I. laxevolutum (Fontannes). Jurassic. 

 Sides flattened; costse bifurcating, rarely trifurcating, with inter- 

 calated single costse; ventral interruption slight, the ends of the 

 costse sometimes overlapping, sometimes continuous. 



Kimeridgian of Sierra de la Caja, Mexico; also Europe. 



321. I. baldarum (Oppel). Jurassic. 

 Larger than preceding, more compressed laterally, ribs some- 



FiG. 1445. Perisphindes skidegat- Fig. 1446. Aspidoceras acanthicuniy 



ensisy section, X z^* (After Whit- suture. (After Burckhardt.) 



eaves.) 



what narrower, but in general character and in the ventral inter- 

 ruption much like the preceding. 



Kimeridgian of Santa Rosa, Mexico, and of Europe. 



CXLIL Aspidoceras Zittel. 



Discoidal ammonites, with large umbiHcus, and more or less 

 flatly rounded venter ; early whorls costate, later ones with two rows 

 of tubercles at the ends of the costae which do not extend across 

 the venter; suture with broad lobes and saddles, not strongly 

 incised. Upper Jurassic. 



322. A. acanthicum (Oppel). (Fig. 1446.) Jurassic. 

 Large, with strongly rounded, smooth venter ; costse fine, numer- 

 ous; umbilicus large, deep; a series of tubercles on the umbilical 

 border; outer row of tubercles wanting on the last two whorls. 



Kimeridgian of Mexico. 



323. A. bispinosum (Quenstedt). Jurassic. 

 With two rows of spinous tubercles on the side, those of the 



umbilical row smaller and more crowded. 



Kimeridgian of Sierra de la Caja, Mexico; White Jura of 

 Suabia. 



