PALEONTOLOGY OF IOWA. 513 



extremities ; mesial fold very prominent, rounded above, 

 scarcely reaching to the beak. 



SURFACE marked by six or seven rounded plications on 

 each side of the mesial fold and sinus, which become obsolete 

 before reaching the beak, leaving on the ventral valve a 

 convex, smooth, or nearly smooth umbo ; concentrically 

 marked by close imbricating lines of growth, the entire 

 shell finely punctate. 



This species is very distinct, from any one of the genus known to me, in its arcuate 

 and incurved umbo and beak, and the absence of plications on the upper part of the 

 valve. When imbedded in the shale, and the umbo alone appearing above the wea- 

 thered surface, it resembles very closely the Orthis umbonata of the Hamilton group 

 of New- York. A farther examination, however, shows the lower part of the valve to 

 be furnished with plications, and the area to be of the character of CYRTIA. From 

 having been worn previous to imbedding in the shale, the surface is sometimes free 

 or nearly free from plications. The shell varies in form, and the sinus is sometimes 

 quite narrow. 



Fig. 2 a. Ventral valve of a worn specimen, with surface free from plications, and having 

 a narrow sinus. 



Fig. 2 b. Interior of ventral valve, showing the central septum, and the closed foramen 

 with the tubular opening between the dental lamellae and pseudo-deltidium, 

 which, at its upper extremity, opens just beneath the beak. 



Fig. 2 c. A larger and more perfect individual, showing the broad subangular sinus, the 

 surface plications and atiriculate cardinal extremities. 



Geological formation and localities. In calcareous shale and shaly limestone 

 of the age of the Hamilton group : New-Buffalo ( Iowa), Rock-island ( 111.). 



Cyrtia triquetra ( n. s.). 



SHELL small, pyramidal ; cardinal extremities somewhat 

 salient. Ventral valve extremely elevated, the beak slightly 

 incurved ; sinus deep, angular or subangular, bounded on 

 either side by strong plications : area large, forming a nearly 

 equilateral triangle, the base being scarcely longer than the 

 sloping sides ; foramen narrow, slightly open at base, and 

 closed for half its length by a pseudo-deltidium. Dorsal valve 

 depressed convex, semicircular or semioval, with a strongly 

 elevated mesial fold which is bounded by a deeper groove 

 than those separating the plications. 

 [IOWA SURVEY.] 65 



