GENUS STREPTORHYNCHUS. 68 



I shall endeavor to show, in another place, that the species now refer- 

 red to this genus may be arranged in three natural groups, though it may 

 be questioned whether this limitation can always be determined by the 

 exterior characters of specimens. 



In the Report on the Fourth Geological District (1843, p. 266), I de- 

 scribed, under the Genus Strophomena, three species ( S. bifurcata, S. 

 ardostriata and S. pectinacea) which now prove to belong to the Genua 

 Streptorhynchus. These determinations and descriptions were made from 

 few specimens, but the characters were unlike, and were deemed suffi- 

 ciently constant to entitle them to specific distinction. Larger collections 

 of specimens have enabled me to make more extensive comparisons, and 

 I am now convinced that these forms graduate into each other, and even 

 take a much wider range than was exhibited in the specimens illustrated. 

 I am moreover satisfied that the Strophomena chemungensis of Conrad is a 

 Streptorhynchus, and specifically identical with those just enumerated, 

 having precedence in point of time. 



In the Tenth Report on the State Cabinet (1857), I described Orthis 

 perversa, which belongs to the Genus Streptorhynchus ; and in the Report 

 for 1860, I described Orthisina ardostriata and O. alternata, both of which 

 are of the Genus Streptorhynchus. In the same year, Mr. Billings 

 described the Streptorhynchus pandora of the Corniferous limestone ; an 

 appropriate name, perhaps, when we reflect that this is but another form 

 of a species to which all those just mentioned must be referred. 



More extensive collections have shown that it is quite impossible to 

 accumulate any considerable number of specimens of any one of these 

 types, without encountering variations which lead to other forms lying 

 intermediate to that one and the other most nearly allied form, until 

 finally it becomes impracticable to draw lines of specific distinction 

 between them. 



To begin with the oldest form at present included in this group, the 

 S. pandora, which occurs in the Schoharie grit and Corniferous limestone, 

 we have usually, but not always, a symmetrical form, differing but little 

 ( if at all ) from S. woolworthana of the Lower Helderberg group. We are 

 able to trace this form in the Hamilton group, where, although rarely 

 [ I'aljEontology IV.] 9 



