THE MESOZOIC AND CENOZOIC ECHINODERMATA 

 OF THE UNITED STATES. 



By William Bullock Clark and Mayville W. Twitchell. 



PART I.— THE MESOZOIC ECHINODERMATA. 



By William Bullock Clark. 



INTRODUCTION. 



GEOLOGIC AND GEOGRAPHIC RANGE. 



The remains of Echinodermata ai - e found in Mesozoic deposits at nearly all horizons and 

 at widely separated localities but are much more numerous and characteristic in Cretaceous 

 than in Triassic and Jurassic strata. In several of the Cretaceous formations they are among 

 the most valuable diagnostic fossils, and 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 Isocrinus and Encrinus, the former appearing in the Lower Triassic of 

 Idaho, the Middle Triassic of Nevada, and the Upper Triassic of California, and the latter being 

 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, although they form no 

 considerable proportion of the known American fossil Echinodermata. 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 Ophiurida? and the Stellerida?, specimens having been found in Wyoming, South 

 Dakota, and Utah. The echinoids are much more fully represented than in the Triassic. Sev- 

 eral genera have been recognized, among them Cidaris, Hemicidaris, Pseudodiadema, Stom- 

 echinus, Holectypus, and Pygurus. Most of the specimens are poorly preserved and are in few 

 places 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. 

 Pygurus has been found only in Texas. 



The Cretaceous echinoderms are very numerous in certain areas. A great variety of types 

 is represented and much of the material is splendidly preserved. Many of the species are nar- 

 rowly limited in geologic range and are therefore important as type fossils. 



The crinoids are represented by Uintacrinus, Marsupites, Pentacrinus, and Rhizocrinus, 

 the first named having afforded a great number of remarkable specimens in the Niobrara chalk 

 of Kansas. Springer has made this material the subject of an elaborate monograph, and most 

 of the great museums of the world contain beautiful specimens from the now famous locality 

 in Kansas. 



