LATER MESOZOIC INVERTEBRATE FAUNAS 193 



facies is typically developed in the Fox Hills sandstone at the top of 

 the group but a closely similar fauna occurs at several lower horizons. 



Ripley fauna. — Toward the south in New Mexico the littoral 

 facies of the Montana fauna blends with the Ripley fauna which is 

 well developed in the latest Cretaceous formations of Texas, Missis- 

 sippi, and Alabama, and throughout the . Atlantic coastal plain to 

 New Jersey. The Ripley and Montana faunas have many species 

 in common and many others that are closely related and yet their 

 aspect is unlike because their dominant types are different. In the 

 Montana fauna the genus Inoceramus is very abundant and varied 

 and ammonoids — especially Placenticeras, Baculites, Scaphites, and 

 other evolute types — are abundant while the Ostreidae, Veneridae, 

 Cardiidae, etc., and gasteropoda play an unimportant role. In the 

 Ripley fauna on the other hand ammonoids and Inoceramus are 

 relatively rare and the Ostreidae, Veneridae, Cardiidae, and many 

 types of gasteropoda, including Volutidae, are greatly developed. 

 The Ripley fauna is more varied and luxuriant, so to speak, than the 

 Montana and apparently indicates a warmer, or at least a more 

 favorable cHmate. There was almost certainly direct connection 

 between the areas occupied by the two faunas, but the life conditions 

 were sufficiently different to determine distinct faunal facies. The 

 Montana fauna probably received some of its elements directly from 

 the Arctic, while the Ripley fauna came in from the Gulf of Mexico 

 and the Atlantic. With the connection between the Atlantic and 

 Pacific closed in the Mexican and Central American region as at 

 present, the Gulf stream would give similar conditions and would 

 distribute the Ripley fauna along the coast from Texas to New Jersey. 

 It is noteworthy that the European fauna most closely related to the 

 Ripley is found at Aachen in Germany and that the most natural 

 route of migration, with such a configuration of the continent as is 

 here assumed, would be from the American i\tlantic coast northeast- 

 ward to Europe. 



A peculiar Cretaceous fauna, apparently contemporaneous with 

 the Ripley, has recently been described by Bose' from Cardenas, 

 San Luis Potosi, Mexico. It contains a few typical Ripley species 

 like Exogyra costata and Gryphaea vesicularis, together with many 



I Instituto Geoldgico de Mejico, Boletin No. 24, 1906. 



