76 
GEOLOGY OF THE FOURTH DISTRICT. 
18 . 
1 Orthonota curta. 4. Nucula maotraeformis. 7. Lingula elliptica. 
2. Nucula machasriformis. 5. Avicula leptonota. 8. Lingula oblata. 
3. Cypricardia obsoleta. 6. Cypricardia? angusta. 9. Lingula acutirostra. 
1. Orthonota curta. — Length rather more than twice the height of the shell; posterior 
slope obtusely carinated. This shell is readily known by its straight hinge line, truncated 
posterior margin, and numerous plications extending from the oblique fold or keel to the an¬ 
terior margin. The characters of the genus, as given by Mr. Conrad, are perfectly applicable 
to this shell. One specimen has been found at Rochester, which is the only one, with the 
exception of those at the ore bed. 
2. Nucula mach(zriformis. — The shell is much elongated, and faintly marked with con¬ 
centric strias; beaks scarcely prominent. The elongated form, and scarcely visible striae, 
distinguish this shell from all others of the group. It is always much flattened from pressure, 
and was apparently a fragile shell. It resembles Machcera ( Solecurtus) costata in form; 
and in some specimens, I have seen impressions of a rib extending from the beak, as in that 
shell. 
3. Cypricardia obsoleta. — Beaks very prominent; shell marked with faint lines or folds, 
scarcely striated. Its prominent beaks and rhomboidal form, with (he worn or decayed ap¬ 
pearance of the shell, are its distinguishing marks. 
4. Nucula mactrceformis. — Shell ovate, marked by concentric striae which are stronger 
upon the anterior margin ; a slight fold (perhaps from compression) extends along the posterior 
slope. Height two-thirds the length of the shell. The form and proportions of this shell 
distinguish it readily from others of the group. 
5. Avicida leptonota. — This shell is usually marked by strong longitudinal ribs, and 
crossed by fainter concentric lines ; posterior hinge margin depressed. In many specimens 
the concentric lines are nearly obliterated, and the longitudinal ribs only visible. It occurs 
in considerable numbers, often crushed, and its form nearly destroyed. 
6. Cypricardia ? angusta. — Shell narrow, much elongated, marked by concentric folds 
which are more prominent and fewer in number on the anterior margin. The hinge resembles 
externally the Cypricardia , but the internal arrangement cannot be seen. 
