MITROSPIRA, A NEW ORDOVICIAN GASTEROPOD GENUS 



B}'^ Edwin Kirk 

 Of the United States Geological Survey 



For the past 30 years or more fragmentary specimens of a large 

 gasteropod have been collected and sent in by Geological Survey field 

 parties from Nevada and California. They were found in the 

 Pogonip limestone (Ordovician) of that region. Enough material 

 had accumulated to show that the gasteropod could not be assigned 

 to any known genus, but despite the fact that 30 or 40 specimens were 

 available for stud}^ it Avas not felt that a description of the form was 

 warranted. During the past field season C. R. Longwell, in southern 

 Nevada and the author in central Nevada, made special efforts to 

 collect good specimens of this interesting gasteropod. It is now felt 

 that as good material is at hand as is likely to be found. 



The gasteropod which is here described as Mitrospira longioelli has 

 a wide distribution in the Great Basin region and is highly character- 

 istic of a fairly narrow zone within the upper portion of the Pogonip. 

 This is of Chazyan age and underlies a fauna closely comparable with 

 zone N of the Chazyan of Canada. Where found the gasteropod is 

 usually present in great numbers. Unfortunately it usually occurs 

 in massive ledges of hard limestone, where thousands of weathered 

 sections may be seen but where it is almost impossible to secure good 

 specimens. The shell of the gasteropod is thick and is replaced by 

 crystalline calcite that shatters badly, leaving only the exfoliated cast 

 of the interior. In thinner-bedded limestones specimens as a rule 

 are imperfect, due to weathering. 



MITROSPIRA, new genus 



The genus, as here defined, contains the single species Mitrospira 

 longwelU, new species. Of this form by careful selection from a 

 large number of specimens enough material has been prepared to 

 show all essential structures in detail. It was found in the prepara- 

 tion of the material that grinding away the matrix was the only 

 satisfactory method. 



No. 2819.-PROCEEDINGS U. S. NATIONAL MUSEUM, VOL. 76, ART. 22 



72825—29 ^ 



