PART II.— THE CENOZOIC ECHINODERMATA. 



By William Bullock Clark and Mayyillb W. Twitchell. 



INTRODUCTION. 



GEOLOGIC AND GEOGRAPHIC RANGE. 



The Cenczoic Echinodermata discussed in the succeeding pages, although largely confined to 

 the Eocene, are by no means absent from later deposits. The Eocene forms are very abundant 

 and embrace some of the most diagnostic fossils of that age, but they are less common than 

 in the Cretaceous. They are found at several Eocene horizons on the Atlantic and Pacific coasts, 

 but are more numerous and characteristic in the Eocene of the South Atlantic and Gulf Coastal 

 Plain, where they occur in large numbers. Nearly all that have been collected belong to the 

 group Echinoidea, although representatives of Crinoidea, Asteroidea, and Holothuroidea have 

 been found. Among the echinoid genera recognized are Cidaris, Ccelopleurus, Echinocyamus, 

 Fibularia, Scutella, Mortonella, Periarchus, Clypeaster, Echinanthus, Cassidulus, Breynella 

 Echinolampas, Hemipatagus, Brissopsis, Linthia, Schizaster, Eupatagus, Macropneustes, and 

 Sarsella. 



The Oligocene strata of the South Atlantic and Gulf areas have in few places been satisfac- 

 torily delimited from the Eocene, so that the age of some of the echinoid material can not be 

 determined with certainty. Among the known Oligocene genera of the South Atlantic and Gulf 

 areas are Cidaris, Echinocyamus, Laganum, Amblypygus, Oligopygus, Cassidulus, Echinolampas, 

 Agassizia, Schizaster, and Eupatagus. The great majority of the forms come from Florida. 

 The Oligocene deposits of California have also furnished specimens of Cidaris. 



The Miocene deposits of both the Atlantic and Pacific coasts have afforded a considerable 

 number of echinodernis, chiefly echinoids. The Atlantic coast Miocene contains Ophioderma ( ?) 

 .Cidaris, Ccelopleurus, Psammecliinus, Scutella, Mellita, Encope, Cassidulus, Agassizia, Brissus 

 Plagionotus, and Echinocardium. The Pacific coast Miocene has furnished Asterias, Amphiura, 

 Cidaris, Scutella, Dendraster, Astrodapsis, Scutaster, C^easter, and Linthia. 



The Pliocene deposits contain very few echinoderms. On the South Atlantic coast from 

 the Carolinas southward a few forms have been recognized, among them Strongylocentrotus 

 Encope, and Diplothecanthus. On the Pacific coast Dendraster, Astrodapsis, and Schizaster ( ?) 

 are found. The Miocene and Pliocene echinoids of the Pacific coast have been found to be of mok 

 than ordinary value in the determination of geologic horizons. This is due to their limited 

 geologic range and to the fact that, where present at all, they are usually abundant and well 

 preserved. 



The Pleistocene deposits likewise have furnished very few echinoderms, and those for the 

 most part of species living in the adjacent seas. Among those recognized from the Atlantic 

 border have been Asterias, Strongylocentrotus, Mellita, Moira, and Toxopneustes. On the 

 Pacific coast, on the other hand, several species of Strongylocentrotus and Dendraster are 

 found. 



The absence of species identical with those of other continents and particularly of Em-ope 

 renders it impossible to attempt detailed correlation of American and foreign horizons. Genera 

 represented in the main divisions of the Cenozoic in America are with scarcely an exception 

 similar to those typical of the same divisions elsewhere. 



The accompanying tables show the stratigraphic position of the more important American 

 Cenozoic formations, the geologic distribution of the Cenozoic Echinodermata of the United 

 States, and the distribution of eastern Gulf Eocene and Oligocene Echinodermata. 



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