686 PROCEEDINGS OF THE BALTIMORE MEETING 



Conclusions 



Two recurrences of the Tropidolcptus carinatus fauna are recorded in the 

 Upper Devonian of Maryland, one in the upper part of the Portage and one 

 about 600 feet above the base of the Chemung, in the central part of the Upper 

 Devonian area. 



A marked similarity exists in the succession of faunas as well as in the 

 lithological features of the Upper Devonian strata of Maryland and New York, 

 suggesting that these strata were laid down in one basin of deposition under 

 similar conditions. 



Certain differences in the faunas east and west of Wills mountain suggest 

 the possibility of the existence of some form of a low barrier in this locality in 

 Chemung time. 



GEOLOGICAL DISTRIBUTION OF TEE ME80Z0IC AND CENOZOIO ECHINODER- 

 MATA OF THE UNITED STATES 1 



BY W. B. CLARK AND H. W. TWITCHELL 



[Abstract] 



Echinoderm remains are found in America in the deposits of every period 

 from the Triassic to the Recent, but are by far the most significant in those of 

 Cretaceous and Eocene age. In several formations they are among the most 

 valuable of diagnostic fossils, while at a few localities they occur in vast 

 numbers. 



Comparatively few Triassic forms have been found. The most common are 

 crinoid stems, representing the genera Pentacrinus and Encrinus, the former 

 found in both the Lower Triassic of Idaho and the Upper Triassic of Cali- 

 fornia, and the latter confined to the Upper Triassic of California. The 

 echinoids are represented by two species of Cidaris, which are confined to the 

 Upper Triassic of California. In addition to these a few indistinct casts, 

 among them a small, poorly preserved starfish, which has been questionably 

 assigned to the genus Aspidura, have been found in the Lower Triassic of 

 Idaho. 



The Jurassic echinoderms are somewhat more numerous and varied, al- 

 though they do not constitute an important element in the fauna. As in the 

 Triassic, the most common forms belong to the genus Pentacrinus, column 

 joints having been found in Nebraska, South Dakota, Wyoming, Colorado, 

 Idaho, Utah, and California. The asteroids are represented by both the 

 Ophiuridse and Stellerida?, specimens having been found in Wyoming, South 

 Dakota, and Utah. The echinoids are much more fully represented than in 

 the Triassic. Several genera have been recognized, among them Cidaris, 

 Hemicidaris, Pseudodiadema, Stromechinus, Holectypus, and Py gurus. The 

 specimens are in the main poorly preserved and are rarely numerous. The 

 first four genera occur only in California, being found in both the Lower and 

 Middle Jurassic. One species of Holectypus occurs in Texas and another in 

 Montana, while Py gurus has only been found in Texas. 



1 Published by permission of the Director of the U. S. Geological Survey. A mono- 

 graphic study of the Mesozoic and Cenozoic Echinodermata of the United States has been 

 made by the authors. Manuscript received by the Secretary of the Society May 27, 1909. 



