A New Ammonite Fauna of the Lower Turonian of Mexico 203 



ras; they differ principally by the presence of a median row of scarcely 

 perceptible, very small tubercles on the ventral part. Unfortunately the 

 specimens are all very small, therefore we can not know if this row of tu- 

 bercles becomes stronger in larger specimens, or if it disappears com- 

 pletely, or if it persists in larger individuals in the same manner as in 

 juvenile specimens. Hyatt does not mention the existence of a median 

 row of tubercles in juvenile specimens of Metoecoceras. Acanthoceras 

 subvicinale occurs in the Cenomanian of Diego-Suarez. 



Thus we see that in Europe as well as in India and Madagascar there 

 seem to exist forms which resemble the Metoecoceras of America. Prob- 

 ably we have to include in this genus at least Pulchellia Gesliana Petras- 

 check and perhaps also Ammonites Geslianus d'Orbigny. Very similar is 

 also Acanthoceras subvicinale Boule, Lemoine et Thevenin, especially on 

 account of the suture, while Acanthoceras vicinale Stoliczka, although 

 showing a very similar ornamentation, differs by its more complicated 

 suture. But we should note that the suture changes considerably in the 

 different Metoeceras of America, that of M. Whitei being much more com- 

 plicated than that of M. Swallovi (in the limitation of Hyatt). At pres- 

 ent we can not decide how many forms should/ be included in the genus 

 Metoecoceras, but can only indicate that this genus is probably represented 

 in the Cenomanian of Europe and perhaps of Madagascar, and that similar 

 forms but with a much more complicated suture, are found in the Ceno- 

 manian and Turonian of India. 



METOECOCERAS aff. WHITEI Hyatt 







PL 12 figures 4, 7 



1876: Buchiceras svallovi White, Inv. foss. Nevada, Utah, etc., p. 202, pi. 20. 

 1893: Buchiecras svallovi Stanton, Colorado Form., p. 168, pi. 37, fig. 1 (?pl. 38, 



figs. 1-3). 

 1903: Meloicoceras Whitei Hyatt, Pseudocer., p. 122, pi. 13, figs. 3-5, pi. 14, figs. 1-10, 



fig. 15. 



In the lower bed of Cerro del Macho occur two fragments of cephalopods 

 which greatly resemble the group of Metoecoceras Whitei: they are espec- 

 ially similar to specimens figured by Hyatt. Our individuals show the fol- 

 lowing features : 



Shell discoidal, very involute, with whorls of nearly rectangular cross 

 section, much higher than broad. The flanks are very little convex, nearly 

 flat, the ventral portion is flattened. The umbilicus is extremely narrow. 

 The ornamentation consists of strong ribs which begin on the umbilical 



