B. T. Hill — Cretaceous Formations of Mexico. 311 



division has escaped the observation of the other writers. Not 

 only has the mountain limestone of Mexico these two subdi- 

 visions but I have found all the broader divisions of the Co- 

 manche Series represented in it. 



The so-called "Hippurites limestone," however, may be 

 accepted as a datum from which to trace the sequence of the 

 overlying and the underlying horizons as shown in the table 

 on page 824. Heilprin and other writers describe its charac- 

 teristic fossils as consisting of the " Hippurites," Radiolites, 

 and Nerineas — genera which do not range higher in the Co- 

 manche Series than the Caprina limestone of Shumard with 

 which horizon the Mexican " Hippurites" limestone is largely 

 identical. 



The Tehuacan or Monopleura beds which Felix and Lenk 

 show underlying the "Hippurites" limestone in Puebla are a 

 distinct lower horizon of the Comanche series to which the 

 writer has recently called attention in the Texas region under 

 the name of the Glen Hose beds of the Trinity division. * 

 From the lower bed they report twenty-eight genera of inver- 

 tebrates (forty-six species) characterized by the corals. Pleu- 

 rocenia, Cladophyllia and Astrocenia ; a single echinoderm 

 (Cyphosoma) and the Mollusca : Monopleura, Nerinea, and 

 Tylostomas of the Natica pedernalis type. All these genera 

 are abundantly found in the Texas Cretaceous and with the 

 exception of Cyphosoma only in the Fredricksburg and Trin- 

 ity or lower divisions of the Comanche Series and are its 

 characteristic distinguishing forms. In both Texas and Mexico 

 they occur below the "Hippurites" or Caprina limestone, as 

 described in Mexico by Felix and Lenk, and under similar 

 stratigraphic conditions except that in Mexico the limestones 

 have been more altered by mountain movements. The mol- 

 luscan genera are all strongly characteristic of the Neocomian 

 of Spain and Portugal. These Monopleura beds are near but 

 not quite at the base of the Mexican Cretaceous ; northward 

 they outcrop in several places. The writer has seen them ten 

 miles east of Bustamente, in Nuevo Leon, on the Tropic of 

 Cancer, and in the Santa Rosa mountain mass of Coahuila. 

 ]STear the first mentioned locality about ten miles to the north- 

 east in the valley of Miquehuana they are underlain by a series 

 of still lower beds between which there is no stratigraphic 

 demarcation. These lower beds consist of alternating strata 

 of arenaceous calcareous clays becoming pack sands with 

 quartz pebbles towards their base and rest unconformably upon 

 the early Mesozoic red beds. These basement strata of the 

 Comanche Series in Mexico strongly resemble in composition, 



* The Comanche Series of the Texas Arkansas Region, Bull. Geological Society 

 of America, vol. ii, pp. 503-528. 



