PALAEONTOLOGY OF IOWA. 515 



Atrypa aspera, var. occidental is, 



PLATE VI. FIG. 3 a, b, c, d. 



Terebratula aspera : SCHLOTHEIM, Pctrefaetenkunde, pa. 263, pi. xviii, f. 3. 

 Atrypa spinosa : HALL, Geological Report Fourth District New- York, 1843. 



This species, as it occurs in Iowa, presents some variations from the prevailing 

 form in New- York and elsewhere in eastern localities. The ventral valve is but slight- 

 ly convex near the beak, concave and deeply sinuate in front; beak very small, per- 

 forate and closely incurved over the opposite; ventral valve, extremly gibbous or 

 ventricose in old specimens, emarginate in front : surface marked by ten or twelve 

 strong dichotomizing plications upon each valve, crossed by strong thickened 

 concentric laminae of growth, which are elevated at intervals into spinelike 

 processes. The young shells have the valves often nearly equal, the inequality in- 

 creasing by age till it becomes very extreme. The number of plications is only about 

 half as many as in full-grown specimens of this species in the shales of the Hamilton 

 group of New-York; and the entire shell, in all its stages of growth, is coarser and 

 stronger. The western specimens approach more nearly the European forms of this 

 species, judging from those I have seen, which arc intermediate in form and in strength 

 of plications. 



Fig. 3 a. Dorsal valve of a specimen below the medium size. 

 Fig. 3 b. Dorsal valve of a very ventricose specimen. 

 Fig. 3 c. Ventral valve of the same. 



Fig. 3 d. Profile of the same specimen, showing the extreme convexity of the ventral 

 valve, and small elevation of the other valve near the beak. 



Geological formation and localities. In calcareous shale of the age of the 

 Hamilton group : New-Buffalo, Davenport, and Independence, Iowa ; Rock- 

 island, Illinois, etc. 



Atrypa reticnlaris. 



PLATE VI. FIG. 4 a, b, c; and FMJ. 5 a, b, c. 

 Anomites reticularis : LINNE, 1767. 

 For synonomy and references, see Palaeontology of New-York, Vol. ii, p. 72. 



SHELL depressed suborbicular in its young state, becoming 

 gibbous and sinuate in its mature condition ; hinge line often 

 nearly straight, and almost equalling the width of the shell ; 

 valves nearly equally convex in the young state, the dorsal 

 valve becoming more gibbous as the shell advances in age, 

 and sometimes acquiring an undefined mesial lobe down the 

 centre. The ventral valve, in the young state, has the beak 

 nearly straight and perforate at the apex, becoming incurved 



