PALAEONTOLOGY OF IOWA. 605 



plenus, like that figured on Plate xiii, fig. 4 e, which occur in the same association, 



may have been regarded as the ventral valves of this species, and thus induced the 



reference to S. sowerbyi. 



Fig. 1 a. Dorsal view and area of an entire specimen. 



Fig. 1 b. Profile view of the same. 



Fig. 2. View of ventral valve of a specimen as it ordinarily appears in the limestone. 



Fig. 3. Interior of a fragment of the ventral valve, showing the muscular and vascular 



impressions, with the low curving dental lamellae. 

 Fig. 4. Cast of the ventral valve, showing the form of the muscular impression and 



direction of the dental lamellae. 

 Fig. 5. Another similar specimen, showing a smaller muscular impression. 



The hinge-line in both these casts is proportionally more extended than in 



the specimen fig. 1 a, b ; but the external features are similar. 



In two other specimens, where the remaining shell is similar to these, the 



hinge-line has a straight extension of more than four inches, with muscular 



impressions similar to those figured, except that they are proportionally 



shorter and smaller in all respects. 



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

 lington and other places in Iowa ; Quincy, Jersey landing, and elsewhere in 

 Illinois. 



Euomphalas latHS ( n. s.). 



PLATE XII. Fio. 7 a, b. 



SHELL discoid : volutions about four or five, very gradually 

 enlarging from the apex ; spire very little elevated ; upper 

 surface of the volutions obliquely flattened ; outer margin 

 distinctly angulate and subcarinate ; inner margin abruptly 

 elevated, and somewhat carinate at the suture line : base 

 broadly umbilicate. 



All the specimens examined are so much worn or distorted, that I have not been 

 able to determine the true form of the aperture. 



This species is distinguished from others in the same association, by the strongly 

 angular form of the upper side of the volutions, and the subcarinate suture margin. 



Fig. 7 a. Upper side of a worn specimen, showing the angular outer margin ( the inner 

 volutions imperfect). 



Fig. 7 b. Lower side of another specimen, showing the broad shallow umbilicus. 



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

 lington, Iowa ; Quincy, Illinois. 



