214 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol.Y. 



mens in tlie Carboniferous series, and their wide geographical separation 

 from other known examples of the genus, it is probable that they belong 

 to an undescribed species ; but the condition of the specimens will not 

 allow of the determination of that question. 



Probably no fossil form is more generally regarded as distinctively a 

 Subcarboniferous type than Archimedes; but the exam]3les in question, 

 according to Br. Peale's observations, seem to have come from strata 

 higher in the series than might reasonably be referred to the Subcar- 

 boniferous group. Unfortunately, the only known associated fossils are 

 some fragments of a Hemipronites, apparentlj^ R. crenistria Philhps sp., 

 a Cyathophylloid coral, probably Zai)hrentis, and a few crinoid segments. 

 These give no definite indication as to the age of the strata, whether 

 Subcarboniferous or Coal-measure, from which the fossils here noticed 

 were obtained, because S. crenistria occurs in both Subcarboniferous 

 and Coal-measure strata in the Mississippi Yalley, and the other forms 

 ^re also common to both groups. 



Genus Ptilodyctia Lonsdale. 



■«=tilodyctia triangulata White. 



Certain forms from the Carboniferous rocks of Arizona are described in 

 this paper, which, although closely related to certain others from the 

 Mississippi Valley, are, nevertheless, regarded as sufficiently different to 

 warrant the bestowal upon them of separate specific names. There are 

 also other instances of similar — and even much closer — relationship be- 

 tween forms occurring in distantly-separated regions, which, together 

 with the former, might be supposed to be due to causes connected with 

 their geographical distribution and their respective regional environ- 

 ment. Such causes have doubtless produced much effect uiDon many 

 species ; but there are others which seem to have undergone little, if any, 

 change, although exposed to all the influences to which the others have 

 been subj ected. Thus, the identity of many Carboniferous species, which 

 occur in the eastern, middle, and western portions of North America, is 

 unquestionable, not to mention the well-known identity of several Eu- 

 ropean with American Carboniferous forms, even mth some of those 

 found in the Western Territories. 



An exam]3le of the wide distribution of American Coal-measure forms 

 is found in Ptilodyctia angulata. The type-specimens of this species 

 were obtained from the Coal-measure strata at Danville, 111., by Mr. Wil- 

 liam Gurley, and describod by me in Proc. Acad, lis'at. Sci. Philad. 1878, 

 p. 35. The examples here noticed were obtained from about the middle 

 of the Carboniferous series on Yampa Plateau, ISTorthwestern Colorado. 

 These examples show that the corallum is ramose, as it is was supi^osed 

 to be when the original description was written, although that fact was 

 not clearly shown by the tyi)e-specimens. The characteristics of the 

 Colorado examples are in all resx)ects like those of the type-specimens, 



