PLATE CXIL 
Orthoceras rudicula. 
Page 268. 
See Plate 37. 
Pig. 1. A longitudinal section of the lower portion of the specimen figured on plate 37, showing the traces of 
an expanded siphuncle, and the organic deposit on the concave sides of the septa, represented 
in the figure by. a darker shade. Wherever there is a separation of the air-chambers at the 
septa, the origin of the raised areola around the insertion of the siphuncle is seen to be due to 
the organic deposit on the peripheral portions of the septa. 
Fig. 2. A septum of the preceding individual, showing the raised areola, with a furrowed margin, produced 
by the organic deposit on the concave side of the septum. The deposit is continued over 
nearly the whole surface of the septum, and presents little variation in its appearance. Upper 
Helderberg limestone. Stafford, N. Y. 
Orthoceras sirpus. 
Page 269. 
Fig. 3. An individual preserving the chamber of habitation and four air-chambers, showing the depth of 
the chambers, and a very , slight constriction of the tube at the aperture. Traces of the 
transverse striae of the surface are also shown. Upper Helderberg limestone. Columbus, 0. 
Fig. 4. A specimen in the same association with the preceding, preserving- the test over the entire surface, 
showing the regular, rounded, transverse striae and the gradually enlarging tube. 
Orthoceras varum. 
Page 259. 
See Plate 79. 
Fig. 5. Lateral view of a specimen, showing a common appearance of the aperture, due in part to compres¬ 
sion. The shell is naturally fusiform, and the chamber of habitation somewhat gibbous- 
Schohai-ie grit. Schoharie, N. F. 
Fig. 6. A septum of the preceding, showing the transverse section of the tube, and the small size and cen 
tral position of the siphuncle. 
Orthoceras Thoas. 
Page 261. 
See Plates 41, 78 B, 79, 80. 
Fig. 7. A large and very much compressed individual, showing a slight curvature of the tube, with the dis¬ 
placed siphuncle, indicated by the raised and curved longitudinal ridge. The maceration of 
tue shell and the destruction of the septa has probably allowed the siphuncle to fall against the 
interior walls of the tube, and the extreme compression has shown it in relief on the exterior. 
Upper Helderberg limestone. Caledonia, JY. Y. 
Fig. 8. An enlargement of the surface to three diameters, showing the sharp, continuous, longitudinal 
ridges, and the finer transverse striae crenulating their summits. Upper Helderberg limestone. 
Dublin, 0. 
Orthoceras inoptatum. 
Page 267. 
See Plate 37. 
Fig. 9. A fragment preserving its normal form and retaining portions of the test over the surface of the 
tube. Upper Helderberg limestone. Clarence Hollow, N. Y. 
Fig. 10. The surface of the test enlarged to three diameters, showing the regular longitudinal striae and the 
more irregular transverse striae, with finer intermediate lines of growth. 
