254 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



certain g-oneric and family- types, tbat liave heretofore been discovered 

 only in the strata representing the earher of those epochs. Thns Rho- 

 docrinus vesperalis and Cyathocrimts stiUaiivus have their nearest known 

 representatives in the Burlington limestone of the Subcarboniferons 

 series. This is interesting because the crinoidal fiiuna of the Upper 

 Coal IMeasures had hitherto presented a good degree of contrast with 

 corresponding faunjie of the diiferent divisions of the Subcarboniferons 

 group as well as with that of the group as a whole. For example, as has 

 been already mentioned, there is a great preponderance of the Cijatho- 

 crinidcc in the Upper Coal Measure strata. These are mostly of peculiar 

 types, and their bodies are mostly also composed of massive pieces. 

 Erisocrinus is peculiar to this latest of the Carboniferous epochs, as rep- 

 resented by the strata of the great Mississippi Valley, and it is interest- 

 ing- to note that the new genus LccijtMocrinus agrees with it in excluding 

 the whole of the anal series of its pieces from participating in the struct- 

 ure of the calyx. 



The other species of crinoids which are named in the list as associ- 

 ated with these new forms bel<)ng to types, either generic or interge- 

 neric, which have been hitherto found only in Upper Coal Measure strata. 



The spines of the species here described as Arcluvocidaris dininnn 

 give a very inadequate idea of the characteristics of the wliole animal, 

 and such a description has very little value in zoological classification; 

 but for the convenience of geological study it is thought best to give 

 systematic names even to such zoologically imperfect objects as these, 

 that they may be used in the classification of all the recognizable fos- 

 sils which characterize different formations respectively. The species 

 represented by these spines has quite a wide geological range in the 

 Upper Coal Measures of the valleys of the Lower Missouri and Upper 

 Mississippi Eivers, and their characteristics are such that the species may 

 be readily recognized. 



Tlie full Carboniferous series of the great Eocky Mountain region is 

 several thousand feet in thickness ; and the horizon within this limit, 

 from which the coral herein described as Acervularia adjunctiva comes, 

 is not accurately known. This discrepancy, however, is apparently of 

 less importance than it otherwise would be, from the fact that not only 

 is the great Carboniferous series of that region not marked off into 

 epochal groups in the same manner that it is in the Mississippi Valley, 

 but it is there everywhere difficult to find any recognizable planes, 

 either paleontological or stratigraphical, for the separation of the series 

 into any well-defined groups. 



