A New Ammonite Fauna of the Lower Turanian of Mexico 231 



species is distinguished by a moderately broad and coiled beak which slopes 

 quickly toward the anterior margin; this steep slope continues on the 

 valve itself, while toward the posterior margin the valve slopes in a very 

 regular curve. The ornamentation consists of radial, thick, and not very 

 numerous ribs. Where these are crossed by specially strong concentric 

 lamellae of growth, a kind of prolongations is formed which nearly re- 

 semble spines. The shell itself is very thick. 



The specimen does not seem to be very nearly related to the Exogyra 

 with radial ribs, of the Mexican middle and upper Cretaceous (Ex. Whit- 

 neyi, Ex. costata, etc.) but much more to the varieties with few ribs be- 

 longing to the group of Ex. Olisiponensis from the border of the Med- 

 iterranean. 



Our fragment is especially similar to an individual from the Cenoma- 

 nian of Wadi Am Rimpf figured by Fourtau 1 ; this specimen shows a very 

 similar ornamentation. 



Very similar also are some specimens from Deba Habe in Nigeria figured 

 by Woods ; these show particularly the strongly sloping region of the shell 

 toward the anterior margin. 



Exogyra olisiponensis occurs in the Cenomanian as well as in the Turo- 

 nian. Many authors certainly take the species in a very wide sense and 

 we do not yet know if it is possible to separate specifically the form of the 

 Cenomanian from that of the Turonian. In general it seems to me that 

 our specimen resembles more the varieties figured from the Cenomanian 

 (compare among others Lartet, Geol. de la Palestine, pi. 11, fig. 1) than 

 those from the Turonian. With this view the layer in which our specimen 

 has been found coincides well; it occurs in the upper part of the marly 

 limestones directly below the Salmurian of Cerro del Macho. We have 

 considered these marls with some doubt as upper Cenomanian. Unfortu- 

 nately we do not know the exact stratigraphic position of the individuals 

 from Nigeria. Woods presumes that they come from the Turonian, be- 

 cause in Gongila, in Nigeria, not only typical cephalopods of the Turonian 

 (Vascoceras, Pseudaspidoceras, bi-carinated Hoplitoides) have been found, 

 but also specimens of Exogyra olisiponensis, while in Deba Habe only 

 Ex. olisiponensis has been collected. A solution of this problem is im- 

 possible for the moment, but there is the possibility that at Deba Habe the 

 Cenomanian might be represented by beds with Ex. olisiponensis. 



Number of specimens: 1. 



Age: Upper Cenomanian (?), upper part of the lowest horizon 6f 

 Cerro del Macho. 



'Fourtau. Faune cret. d'fegypte, p. 287, fig. 5. 



