GEOLOGICAL AND GEOGEAPH10AL DI8TBTBUTION. 135 



types, while we strongly suspect that G. cornutus is a representative of our now type 

 Heteroblostus {?\. VI. fig. 1). 



Schizoblostus is fairly abundant in the Burlington Group, and, so far as our infor- 

 mation goes, one species occurs in three of the other subdivisions of the American 

 Carboniferous Limestone, the Chester Group having none. Metablctstus is represented 

 in all but the lowest, and Pentremites Grosvenori, Shumard, is possibly congeneric with 

 the Silurian Troostocrinus Reinwardti. The allied genus Tricoelocrinus, however, 

 is peculiar to the Keokuk and St. Louis Groups. 



Our knowledge of the American Carboniferous Codasteridoe is limited to four 

 species in the Burlington Limestone and two undescribod species of Phcenoschisma 

 from New Mexico. Two of these four belong to Orojphocrinus, while the other two 

 may be either Codaster or Pkamoschisma ; but in neither case have we any certain 

 information respecting the presence or absence of hydrospires in the anal interradius, 

 though we suspect that Cadaster Whitei, Hall, is rightly so named. The internal cast 

 described by Miller 1 from the Keokuk Group, under the name of Codaster graUosus, 

 seems to have no indication of the presence of any external hydrospire-slits, and cannot 

 therefore be referred to this genus. 



The American Carboniferous rocks thus contain ten distinct genera, and two 

 doubtful ones, as against the ten well-defined generic types which occur in Britain. 

 Pentremites and perhaps Schizoblastus are most abundant in America, Granatocrinus 

 and Mesoblastus in Britain; but they are far more rare, both specifically and 

 individually, than the two leading American genera. The Troostoblastidse, which 

 are fairly common in the American Carboniferous, are absent in Britain ; while, 

 on the other hand, there are three Devonian species in Europe, but none in 

 America. 



It will be seen from the stratigraphical list on pp. 137-144 that the distribution 

 of the various species of Blastoids is very limited both in Space and in Time. A few 

 species appear to be common to the Upper and Lower Devonian of America ; but 

 each of the great divisions of the Subcarboniferous in the Mississippi valley seems 

 to have its own particular types. No Blastoid occurs on both sides of the Atlantic ; 

 one species is common to the Devonian of Spain and Germany ; and another to the 

 Carboniferous Limestone of Britain and Belgium. But with these exceptions the 

 range of individual specific types is very limited indeed. 



The following Table gives a convenient resume of the facts detailed above. It 

 does not include the problematical Blastoidocrinus from the Lower Silurian of 

 Canada and Bussia. 



1 "Description of two new species from the Niagara Group and five from the Keokuk Group." Journ. 

 Cincinnati Soc. Nat. Hist. 1SS0, vol. ii. p. 215, pi. xv. fig. 5. 



