ag 
34 F. B. Meek and F. V. Hayden 
On the left side of the shell, the anterior extremity of the mus- 
cular impression (a, fig. 2 and 3, of Plate I.) is generally not 80 
much enlarged as on the right, but sometimes extends slightly — 
farther forward; posteriorly it passes around in the form of a ~ 
a band to the middle of the slope behind (8, fig. 2 and 3), — 
where it is abruptly enlarged and curves upwards. From this 
cies A. borealis, and an undescribed form, there are six equidis- 
ines on some of the others, presenting the same inter- 
nal characters, we infer they can scarcely be regarded as a ge 
neric character. - 
_ The more convex species, such as’ A. borealis, are somewhat 
similar externally to some species of Hipponyx, but to say noth- 
