140 University of Texas Bulletin 



the top by a shallow indentation and constricted near the base. The 

 external and the two lateral saddles are very similar in form, the first 

 one being the highest, while the others rapidly decrease in length. 

 Their width is nearly the same. The second auxiliary saddle is much 

 broader and lower than the rest. The internal sutures are unknown. 



Dimensions : 



Diameter 19.9 mm (1) 



Width 14.7 0.74 



Height of last whorl 10.0 0.50 



Diameter of umbilicus 0.8 0.04 



Relation to other species: 



Our species is very similar to Marathonites J. P. Sniithi n. sp., but 

 differs in many respects. It is still more involute, the flanks are a 

 little more flattened, the umbilicus is much narrower and the septa are 

 much nearer together. A great difference exists in the form of the 

 trifid lobes. While the points are nearly equal in length in our species, 

 the middle point is very prominent in M. J. P. Smithi. Also saddles 

 and lobes are generally broader in our species and especially the first 

 auxiliary saddle is much wider and lower. The transversal constric- 

 tions in our species are much deeper and apparently also more 

 numerous. 



M. vidriensis n. sp. is easily distinguished from the present species 

 by its much less involute form and wider umbilicus, the flatter flanks 

 and the less rounded ventral portion. The suture is also different, the 

 trifid lobes showing a very prominent middle point, and the first aux- 

 iliary saddle being nearly of the same form and size as the external 

 and the two lateral saddles. 



M. Hargisi n. sp. has a somewhat similar but more globose form 

 and its suture is entirely different. 



Stacheoceras Ganti Smith-^ of the Pennsylvanian is much more 

 involute and lobes and saddles are much broader and shorter. 



Stacheoceras Parkeri Heilprin^ from the Pennsylvanian seems to 

 be somewhat similar in form and the septa are also very near each 

 other, but the suture is entirely different, the first lateral lobe being 

 bifid. 



'Smith, Carb. Amm. of America, p. 132, pi. 21, fig. 14-16. 

 ''Heilprin, Proc. Acad. Nat. Sci. Phila., 1884, p. 53, fig. 1, 2. 



