1046 



NEW YORK STATE MUSEUM 



S. p e t i 1 u s of the Waldron Siluric beds is succeeded 

 genetically, though not chronologically, by S. crispus sim- 

 plex, and this in turn by the normal S. crispus. There 

 is also a strongly accelerated type in the Waldron beds which 

 is characterized by a depression in the fold and rarely an eleva- 

 tion in the sinus, and by angular plications. 



The normal New York type of S. crispus has strongly 

 rounded but narrow plications, with a moderately deep, rounded 

 sinus, which broadens strongly toward the front. This species 

 was undoubtedly derived from the Waldron S. crispus by 

 migration. Whether the pauciplicate rhombic type above 

 mentioned is due to a direct immigration or to retardation of 

 the normal S. crispus of New York can not be readily 

 determined. The same may be said of S. bisulcatus Van- 

 uxem, which is still more primitive, or more retarded. The 

 results of our present studies in the phylogeny of these species 

 may be summed up in the annexed diagram. 



