OBSERVATIONS ON THE GENUS NUCLEOSPIRA. 



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

 the name Spiriferl 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, according- 

 ly, in the second volume of the Palaeontology of New- York, de- 

 signated the Niagara fossil Orthis pisum. 



Since that period, my collections from the Helderberg have re- 

 vealed 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 unequivocal charac- 

 ters. 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 cor- 

 respond 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 pre- 

 sence of a spire sufi&ciently distinguishes it from the latter. 



The Lower Helderberg group furnislies one, and perhaps two, 

 other species; and I find that the fossil described by me as Atrypa 

 concinna in the Report of the Fourth Geological District (1843), is 

 another species belonging to the same group of fossils, in which both 

 the external characters and internal structure differ so essentially 

 from any of the described genera of Brachiopoda as to constitute a 

 distinct genus; and which, from the general nucleolar character of 

 the known species, I propose to designate Nucleospira. 



