PALAEONTOLOGY OF IOWA. 597 



This is one of the species usually referred to O. michelina ; but it differs in many 

 respects from others which are so included. Compared with a European specimen of 

 that name in my collection, it has a more extended beak and higher area in the 

 ventral valve, while it corresponds in surface markings. 



Fig. 4 a. Ventral valve of a specimen of medium size. 



Fig. 4 b. Enlargement of striae, showing the tubular openings upon the surface. 



Geological formation and localities. In the Burlington limestone : Bur- 

 lington, Iowa ; Quincy, Illinois ; Hannibal, Missouri. 



Ortliis swallovi ( n. s.). 



PLATE XII. FIG. 5 a, b. 



SHELL somewhat semiglobose : hinge-line less than the 

 greatest width of the shell ; cardinal extremities rounded. 

 Ventral valve depressed-convex near the beak, flattened at 

 the sides, with a broad depression beginning in the middle 

 and becoming more strongly marked towards the base : area 

 large, extending to the extremities of the hinge-line ; fora- 

 men broad. Dorsal valve very convex, gibbous or somewhat 

 ventricose in the middle and on the umbo, broadly curving 

 in front and sloping more abruptly at the sides. 



SURFACE marked by fine closely arranged striae, some of 

 which become enlarged and present numerous tubular open- 

 ings upon the surface ; concentrically marked by numerous 

 unequally distant, sharp, imbricating lines of growth. Entire 

 substance of the shell punctate. 



This is one of the largest and finest species of ORTHIS in the Carboniferous lime- 

 stones, which I dedicate with pleasure to Prof. SWALLOW, in whose collections from 

 the Missouri Survey I have seen much finer specimens than any yet furnished from 

 Iowa localities. 



This species resembles the Orthis resupinata of Europe ; but on comparison with 

 several specimens of that name in my collection, it shows a more extended hinge-line 

 and larger area : the ventral valve is less convex than in a specimen of O. resupinata 

 from Yorkshire, England, which has the dorsal valve equally ventricose with ours; 

 while an Irish specimen of the same name shows no depression in the middle of the 

 ventral valve. A third specimen under the same name differs from cither of these. 



Fig. 5 a. Ventral valve of a mature individual of this species. 

 Fig. 5 b. Dorsal valve of another specimen. 



Geological formation and localities. In the Burlington limestone : Bur- 

 lington, Iowa ; Quincy, Illinois, and numerous localities in Missouri in the 

 same formation. 



