(JAEBONIFEROUS PERIOD. 135 



T>'hich is wide at the front, and distinctly defined from front to beak ; area 

 of moderate width, concave, the beak being strongly incurved over it. 



Dorsal valve having a mesial fold corresponding in shape and distinct- 

 ness with the srinus of the other valve ; beak not prominent, projecting very 

 little over the cardinal border. 



Surface of each valve bearing from twenty-four to thirty radiating pli- 

 cations, distributed over the sides and also upon the fold and sinus ; the 

 middle plications somewhat larger than any of the others, the size diminish- 

 ing gradually toward the postero-lateral regions. 



The numerous specimens of this species in the collections present most 

 perplexing variations, and I have repeatedly been almost persuaded to 

 arrange some of them under a separate specific name, but the presence of 

 inteimediate forms has prevented such a decision. I have no doubt of the 

 specific identity of our shell with S. Rockymontanus Marcou, and it will also 

 be noticed that some of our examples were obtained from near the same 

 localities that furnished the type-specimens u^sed by Professor Marcou in 

 writing his description. Many of the examples under examination agree in 

 all essential particulars with 8. oplmus Hall, which thus becomes a synonym 

 of S. Bockymontanus. Compared with figui-es and descriptions of S. hisulcatus 

 Sowerby from the Subcarboniferous strata of G*reat Britain, some of our 

 examples agree so closely that I suspect this American shell may, after all, 

 prove to be identical with that European species. 



Position and locality. — Strata of tlie Carboniferous period : North P^ork 

 of Lewiston Caiion, Oquirrh range, and upon the west face of Oquirrh 

 range, Utah ; also near Santa Fe, New Mexico. 



Subgenus MAETINIA McCoy, 1844. 

 Spirifer (Martinia) planoconvexus Shuinard. 



Plate X, tig. :5 a, h, and c. 



Spirifer planoconvexus Sliumard, 1855, Geological Report of Missouri, 202. 

 Amhococlia gcnimula McOiiesney, 1860, Descr. New Paleozoic Fossils, 41. 

 Spirifer planoconvexus Meek and Hayden, 1864, Paleont. Upper Missouri, 20, 21. 

 Spirifer planoconvexus Geinitz, 1S66, Carbou formation und Dyas in Nebraska, 42. 

 Compare with Spirifer Urii Fleming, 1828, British Animals, 370. 



Shell very small ; breadth varying from a little more to a little less 

 than the length ; hinge-line of considerable length, but always shorter than 



