PLATE XXVIII. 
1. Orthocera Fusiformis. (Spindle-shaped Orthocera.) Arcuated, fusiform, smooth, round, tapering towards 
smaller end; siphunculus central; outer chamber large. 
2, 3. 0. Circularis. (Circular Orthocera.) Nearly cylindrical or tapering; partitions thin, concave, ap¬ 
proximate, about one-sixteenth of inch distant, with edges even; siphunculus situate midway between centre 
and margin; diameter of large end 1 in. 
4. 0. Cincta. (Girdled Orthocera.) Elongate, almost cylindrical; surface with numerous sharp, undulous, 
annular striae ; siphunculus central; septa concave, distant; section slightly ovate. 
5. 0. Breynii. (Breyn's Orthocera.) Cylindrical, elongate, tapering ; septa numerous, ovate, oblique, con¬ 
cave, approximate, shallow; siphunculus situate in one focus between centre and margin; outer shell thin, 
plain; section elongated, oval. 
6,7. 0. Conica. (Conical Orthocera.) Elongated, conical, smooth ; aperture ovate, chambers many ; septa 
with even margins and regularly concave surfaces ; siphunculus small, oval, close to margin. 
8. 0. Rugosa. (Rugged Orthocera.) Sub-cylindrical, arcuated; with annular, undulating, distant ridges, 
crossed by strong striae of knotty appearance, intervening spaces with longitudinal, tuberculated, sub-echinated 
lines ; siphunculus minute, situate close to edge; two chambers between each of transverse annulations. 
9,10. 0. Undulata. (Waved Orthocera.) Oval, tapering, thin, smooth; partitions many, parallel, somewhat 
concave; their edges ascending, oval, with wave on each side, equidistant, siphunculus situate near thicker end, 
about one-sixth of diameter from side of shell; section broad oval. 
11, 12. 0. Annulata. (Ringed Orthocera.) Tapering, sub-compressed, with strong oblique, equidistant 
annulations, and minute, transverse, undulating strife ; space equal to four rings plain near aperture, within 
which siphunculus is situate ; section oval. 
13. 0. Cornu-Ibex. (Ibex-horn Orthocera.) Somewhat arcuated, cylindrical, with many undulous, smooth 
annulations ; intervening spaces smooth. 
14. 0. Laterale. (Lateral Orthocera.) Tapering, smooth, compressed, with equidistant, wide-set, oblique 
septa, sides arcuated ; section ovate. 
15. 0. Steinhaueri. ( Steinhauer's Orthocera.) Circular, very concave, with thin margins, even edged, wide, 
elongated, tapering ; with parallel, regular, transverse, striae ; septa distant; chambers deep ; siphunculus large, 
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situate close to one side ; section circular. 
16. 0. Unguis. (Claiv Orthocera.) Smooth, abruptly arcuated toward thicker end; septa many; section 
orbicular. 
17. 0. Striata. (Striated Orthocera.) Nearly cylindrical, tapering ; surface logitudiually striate ; aperture 
oval, one-third broader than wide; septa very thin, many, wide-set ; chambers deep; siphunculus large, central. 
Greatest known length, 11 in. 
18. 0. Dentaloideum. (Tooth Orthocera.) Greatly curved, tapering, with many small, longitudinal ridges 
l 
and sulci. 
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