PALAEONTOLOGY OF IOWA. 711 



probable that the S. fasciger of KEYSERLING may be identical with the American 

 species. 



Fig. 2 a. Dorsal view of a specimen above the medium size, having the extremities 



imperfect. 

 Fig. 2 b. Ventral view of the same. 



Geological formation and localities. In the Coal-measure limestones and 

 rocks of the same age in Ohio, Illinois, Iowa, Missouri, Nebraska ; Santa 

 Fe and Pimos village, New-Mexico ; and other places in the range of the 

 Rocky mountains. 



This species has perhaps a wider geographical distribution than any of the genus 

 known upon this continent : over much of the area, it is associated with Spirifer 

 lineatus, Terebratula (JUhyrisV) subtilita, and Productus rogersi ; while Terebratula 

 millcpunctata = T. bovidenst has possibly a distribution of equal extent. From our 

 present knowledge, it would appear that this species is known in both the Upper and 

 Lower Coal measures*. 



Spirifer opiums ( n. s.). 



PLATE XXVIII. Fia.l a, b. 



SHELL rotund, gibbous, length and width nearly equal ; 

 hinge-line equalling or sometimes a little greater or less than 

 the width of the shell below : valves nearly equally gibbous 

 in their greatest convexity. Dorsal valve regularly convex, 

 with a strong well-defined mesial fold which is simple at 

 the apex, dividing a little below, and each division again 

 dichotomizing, the two middle divisions stronger than the 

 lateral ones, and separated by a well-defined groove ; in 

 some of the smaller shells, the lateral plications of the me- 

 sial fold are feebly or not at all developed : beak elevated 

 a little above the hinge-line, and incurving over a narrow 

 defined area. Ventral valve most gibbous above the middle, 

 and abruptly rounding towards the sides and front; mesial 

 sinus well defined, simple above, and becoming marked by 

 three small plications in the middle and lower part ; beak 

 much elevated and strongly incurved, covering the upper 

 part of the large foramen ; area high in the middle, slightly 



* The Spirifer striatus is cited as having a wide geographical distribution on this 

 continent, as well as in Europe; but I have not been so fortunate as to see specimens 

 from any distant localities. 



