INVERTEBRATES. 371 



Macrocheilus intercalaris, M. and W. 



PI. 31, fig. 6 a, 6 6. 



Macrocheilus intercalaris, Meek and Worthen, October, 1860. Proceed. Acad. Nat. Sci. 



Philad., p. 467. 

 Macrocheilus pulchellus, Meek and Worthen, Oct. 1860. lb. 



Shell of medium size, rhombic-subovate ; spire conical, 

 forming about one-third the entire length, pointed at the 

 extremity ; volutions six to seven, compressed-convex, increas- 

 ing rather rapidly in size, last one comparatively large, but not 

 ventricose, widest near the middle, and compressed above; 

 aperture narrow subovate, acutely angular above, narrowly 

 rounded and slightly effuse below; outer lip sharp, a little 

 prominent along the middle ; columella somewhat tortuous, 

 moderately callous, and provided with an oblique prominence 

 or obtuse fold near the middle of the aperture, sinuous just 

 above the fold. Surface smooth, but showing under a lens, 

 obscure traces or lines of growth. Length, 1.15 inches; breadth, 

 0.70 inch; length of aperture, 0.70 inch; breadth of ditto, 

 0.34 inch ; apical angle convex, divergence 64°. 



This species differs from the last, not only in being less yentricose, and in 

 having the whorls less convex, but in having the callosity of its inner lip smooth, 

 instead of striated above the middle.- The differences of form are generally 

 more strongly marked than would appear from our figures 5 a and 5 b, since a 

 majority of the specimens of N. rnedialis have the body whorl proportionally 

 larger, and the spire shorter, than in the specimen represented by our figures 

 5 a and 5 b, though that is much the best specimen we have seen. 



We now regard the form we described under the name M. pulchellus, as most 

 probably only a variety of the M. intercalaris. It is exceedingly difficult, 

 sometimes, to separate the species in this genus, since they are generally varia- 

 ble, and are sometimes very closely allied. 



Amongst the specimens sent by Dr. Stevens from Pittsburgh, there are two 

 that we think belong to this species, as they agree well with its type in form 

 and general appearance, while they differ materially from those we regard as 

 M. primly enius, from the same locality. 



Locality and position : Same as last; also from same horizon at Pittsburgh, 

 Pennsylvania. 



