1890.] NATTKAL SCIENCES OE rilll.A DEEl'HIA. 457 



the fornmtion in question represented a transition bed between the 

 Jurassic and Cretaceous systems.^ I am not acquainted with the 

 species to which the form figured^ by Barcena is referred, nor am I 

 able to determine its special Jurassic features. On the other hand, 

 I have identiiied the same species, associated with an abundance of 

 Hippurites and with Nerinoea Castilli (Barcena), in the limestone 

 of the Cerro de Escamela, near the town of Orizaba, whose position 

 in the Upper Cretaceous series (Senonian) is well established. The 

 fragment from the same region which Barcena doubtfully refers to 

 Nerincea GoodhaUl (Jurassic) is much too imperfect for recognition. 

 Ramirez has identified, as he believes, the limestone of Apasco in 

 the Sierra Mojada of Coahuila, which' would carry the formation 

 close on to the Texan frontier. The determination is based largely upon 

 the recognition in both localitiesof an identical species of Hippurite, 

 which the author figures.' The form certainly appears identical 

 with a species which I have myself collected in the southern mountains, 

 and which is also reported from Tancanhuitz, in the State of San 

 Luis Potosi. It seems to me possible, however, that a portion of the 

 Sierra Mojada limestone may represent a somewhat lower horizon, 

 the Cenomanian (=rthe formation of Arivechi, in Sonora), but the 

 data on this ])oint are still obscure. Ramirez enumerates and 



since a local bed, containing some fossil remains less Cretaceous in aspect than 

 those found in the upper beds, has been found to underlie the so-called Lower Cre- 

 taceous. Dr. White, in his valuable paper on the " Lower Cretaceous of the 

 Southwest" (Am. Journ. Science, 1889, 2nd part, p. 440), well recognizes that 

 the Lower Cretaceous of that section of the United States is not the equivalent of 

 the Lower Cretaceous of Europe, and it is therefore the more to be regretted that 

 he makes use of a classification which cannot be of general application. The con- 

 fusion arising from such loose classification is immediately shown in the conclu- 

 sions that are deduced from it. Thus, being Lower Cretaceous (in the American 

 sense), Dr, White seeks for the equivalents of the Comanche series in the Lower 

 Cretaceous of Europe, and it is perhaps not surprising that " we cannot say with 

 confidence that the Comanche series really represents any one of the divisions of 

 the European Cretaceous from the Gault to the Lower Neocomian inclusive" {loc. 

 cit., p. 442). Assuredly not, since the deposits in question lie above the European 

 Lower Cretaceous, and are, as I have shown, not older than the Cenomanian. 



1 Viaje a la Caverna de Cacahuamilpa, p. 17, Me.\ico, 1874. 



2 Datos para el Estudio de las Rocas Mesozoicas de Mexico, p. 12, 1875; 

 Materiales para la Formacion de una Obra de Paleontologia Mexicana, Anales del 

 Museo Nacional de Mexico, 1877, p. 201. 



3 Exploracion en la Sierra Mojada — Anales del Ministeriode Fomento, 1880, 

 pi. l.fig. 1. 



