PALEONTOLOGY OF IOWA. 703 



Fig. 1 a, b. Dorsal and ventral views of a specimen of the ordinary size. 

 Fig. 1 c. Profile view of the same. 



Geological formation and localities. In the Kaskaskia limestone : Chester, 

 Illinois ; and Crittenden county, Kentucky. 



Atliyris subquadrata ( n. s.). 



PLATE XXVII. FIG. 2 a, b, c, d. 



SHELL ovoid, subquadrate, more or less gibbous according 

 to age ; greatest width a little below the middle, curving 

 thence abruptly to the base, and more gradually to the beak. 

 Dorsal valve the more convex, with a broad mesial fold, 

 which becomes more strongly developed below the middle 

 of the shell : beak abruptly incurved beneath the opposite. 

 Ventral valve the less convex ; sinus broad and deep below 

 the middle, and scarcely conspicuous on the upper third of 

 the shell, produced in front : beak incurved, and vertically 

 truncated by a round foramen. 



SURFACE marked by fine concentric strise of growth, and 

 towards the margins by strong imbricating lamellae, which 

 thicken the edges. Certain states of preservation of the sur- 

 face show fine radiating striae, while the perfect condition 

 presents a finely punctate appearance. 



The individuals of this species vary in their characters to a considerable degree; 

 the old shells showing a broad deep sinus which elevates the front of the shell often 

 in an extreme degree, while in some of the younger individuals there is but a mode- 

 rate sinus below, and a slight angular depression reaching to the beak. 



This species is more abruptly and deeply sinuate than Terebratula (.Athyris?') sub- 

 tilita, which it resembles in some respects, though always proportionally broader 

 and shorter. Space does not permit an illustration of the variations presented in 

 different stages of growth. 



Fig. 2 a. View of the dorsal valve, and beak of the ventral valve, of a specimen of 



medium size. 



Fig. 2 b. Ventral view of the same. 

 Fig. 2 c. Profile view of the same. 

 Fig. 2 d. Ventral view of a smaller and more orbicular form, where the shell is 



exfoliated, showing the vascular impressions. 



Geological formation and localities. In the Kaskaskia limestone : Chester, 

 Illinois ; and Crittenden county, Kentucky. 



