[78] 



ft Umbilicus open. 



X Shell rounded. 



5. Mesodon thyroides, Say. 



Plate 8, figure 1. 



Eounded, thin, regularly and closely obliquely striate ; spire 

 depressed, conical, suture distinct, but not deeply impressed ; 

 wborls 5, convex, the body large, well rounded, slightly 

 declining at the aperture, behind which it is a little constricted ; 

 aperture obliquely semilunar, the lip moderately reflected, with 

 a slight transverse tooth or varnish of callus near the top of 

 the parietal wall ; umbilicus open, but partially bounded by a 

 raised acute dilatation of the lips. Uniform light horn or 

 straw color. 



Diam. 25, height 16 mill. 



Inhabits plentifully the whole country east of the Eocky 

 Mountains, but is particularly numerous in the Western States, 

 becoming rare in New England, and partially replaced by M. 

 bucculenta in the South. 



This is one of our most beautiful species, its delicate tex- 

 ture, regular rib-like striae, pleasing color and frequently 

 roseate lip, give it a particularly neat aspect. Until it has 

 become quite mature, the only trace of the parietal tooth is 

 frequently a slight uncolored thickening, appearing as though 

 varnished. In the immature state it is likely to be confounded 

 with young individuals of a small variety of M. albolabris, a 

 species which always inhabits with it. It has also been very 

 generally confounded with M. bucculenta, a smaller, more 

 globose shell, the description of which follows. 



6. Mesodon bucculenta, Gould. 



Plate 8, figure 2. 



Globose, rather thin, with delicate, oblique, regular raised 

 strire ; spire obtusely elevated, suture distinctly, but not 

 deeply, marked ; whorls 5, convex, body large, rounded com- 

 pactly, a very little deflected towards the aperture ; mouth 

 lunate, with a broad white lip, partially covering the umbili- 

 cus ; parietal tooth sometimes scarcely developed, but occasion- 

 ally strong. Eeddish horn color. 



Diam. 16, height 10 mill. 



