LATER MESOZOIC INVERTEBRATE FAUNAS 421 
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 réle. 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 climate. 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 Atlantic 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 
« Instituto Geolégico de Méjico, Boletin No. 24, 1906. 
