ai8 PALiEONTOLOGY OF NEW-YORK. 



NUCLEOSPIRA. 



In Murchison's Silurian System, Mr. Sowerby has (described, under the 

 name Spirifer? pisum, a species differing essentially in general external 

 characters from the typical forms of that genus. This species has been 

 adopted as a true Spirifer in Morris's Catalogue of British Fossils, and 

 in the Nomenclator Palseontologicus of Bronn, as well as elsewhere. 

 Subsequently I discovered in the Niagara shales a form so similar to the 

 British species, that I regarded it as identical ; but, from the condition 

 and character of the specimens, I considered them as more nearly allied 

 to Orthis than to Spirifer, and, accordingly, in the second volume of the 

 Palaeontology of New- York, designated the Niagara fossil Orthis pisum. 



Since that period, my collections from the Helderberg have revealed a 

 species similar to the one from the Niagara group; but among the numerous 

 individuals from the latter rocks, I found several which were clearly 

 furnished with internal spires like the true Spirifer, thus separating it 

 from Orthis by imequivocal characters. Finding no genus for the reception 

 of these forms, I described the latter as Spirifer ventricosa ; and it has 

 been so published in my descriptions of new palaeozoic fossils in the 

 Report of the Regents of the University upon the State Collections of 

 Natural History. 



Farther examination has satisfied me of the impropriety of placing 

 this fossil under either of the genera named, for several reasons. The 

 central depressed line, or narrow sinus, which might be regarded as the 

 mesial sinus of Spirifer, is almost equally a character of both valves ; the 

 apparent area is not a true area ; and the apparent foramen, being merely 

 a depression in the false area, does not correspond to the foramen either 

 of Spirifer or of Orthis, not opening into the cavity of the shell. The 

 hinge line is not extended in the manner of these shells, particularly of 

 the former ; while the presence of a spire sufficiently distinguishes it 

 from the latter. 



The Lower Helderberg group furnishes one, and perhaps two, other 



