SANTO DOMINGAN PALEONTOLOGICAL EXPLORATIONS 227 
rockweeds. A marked feature was the local distribution, certain 
assemblages being limited to certain coves. Univalves far out- 
numbered bivalves. The genera Terebra, Conus, Drillia, Cythara, 
Cancellaria, Oliva, Marginella, Mitra, Strombina, Murex, Cypraea, 
Strombus, Cerithium, Pyramidella, and Turbonilla abounded. 
Among the bivalves was a profusion of Arcas, while the genera 
Chama, Pecten, Cardium, Protocardia, Chione, Petricola, Tellina, 
and Corbula were represented by many beautiful forms. Lignitic 
beds, gravels, and clays were being deposited. As the Middle 
Miocene was ushered in the change of conditions began first to be 
felt by the sensitive corals, then the sluggish molluscs were affected 
and a large proportion of them ceased to exist and were replaced 
by different forms. Members of the Myrtle, Laurel, and Mimosa 
families grew upon the neighboring shores, with a number of woods 
of new species not known from elsewhere. 
The Pacific element in the fauna of the Yaqui Valley is very 
marked in some cases. This fact that the nearest living allies 
and apparent descendants of certain of the fossil species are now 
on the west coast or in the Gulf of California might seem to indicate 
an Oligocene age of the deposits. But if we regard sedimentation 
as proceeding uninterruptedly, there is no reason why the species 
may not have lived on in the Antilles for some time and not suffered 
immediate extinction. Moreover, Dr. Vaughan has been led to 
suggest from the evidence gathered from fossil corals of the Cali- 
fornian region that a trans-Isthmian passage may have existed 
later than Oligocene time. This appears strengthened by Professor 
Harris’ observations of the Pacific and Gulf of Californian affinities 
of the Miocene molluscan fauna of the Galveston deep well. Our 
Antillean Miocene is, however, entirely distinct from the deep- 
well Miocene faunas of Texas and Louisiana, nor has it any resem- 
blance to the cold-water Chesapeake Miocene of Maryland and 
Virginia. 
Certain of the fossils of the Yaqui Valley most closely resemble 
species now living in the deep sea. Undoubtedly, however, the 
fossils were shallow water in habit of life because their associates 
all conclusively prove this; even the foraminifera being all genera 
now characteristic of shallow waters. Thus indications are 
