PLATE XXIII. 
1. Nautilus Astacoides. {Little Lobster Nautilus.) Discoidal, umbilicate; inner volutions wholly concealed ; 
ambit narrowed toward aperture ; sides smooth, concentrically lineated with transverse lines of growth; aperture 
expansive, occupying two-thirds diameter of shell; siphunculus central. 
2. N. Undulatus. (Waved Nautilus.) Gibbous; inner volutions concealed; sides produced, with large 
undulations; edge of back scalloped ; aperture obcordate; siphunculus central; septa numerous, crossed on 
surface by au undulation ; thickness half diameter of disc. Greatest diameter 12 in. 
3. N. Oxystomus. ( Sliarp-mouthed Nautilus.) Lenticular depressed; with four volutions, inner ones half 
exposed ; sides smooth; ambit acute ; septa outwardly concave. 
4. N. Truncatus. (Truncated Nautilus.) Thick; inner volutions wholly concealed; umbilicate; sides 
flattened ; ambit flat; aperture elongated, quadrangular; siphunculus oval, situate nearest inner end of aperture ; 
septa many; thickness not quite equal half diameter. 
5. N. Simplex. {Simple Nautilus.) Spheroidal, depressed; inner volutions wholly concealed; sides plain, 
umbilicate; aperture lunate, with truncated angles, embracing sides of volutions; septa many, flattened; 
siphunculus situate near inner edge of septum ; thickness three-fourths greatest diameter of disc, which is from 
1 in. to 13 in. 
6. N. Excavatus. {Excavated Nautilus.) Globose; largely umbilicate; umbilicus shape of reversed cone, 
producing excavated appearance hi sides; sides smooth, straight; aperture sub-quadrangular, produced at sides 
by convex extremity of umbilicus, arcuated in front; siph unculus central. 
7. N. Sulcatus. {Furrowed Nautilus.) Discoidal, with four almost wholly exposed volutions; sides ventri- 
cose, with two large spiral sulci and two or three shallow, broad, smaller ones on each side ; one of sulci concealed 
in inner volutions and with sharp, carinated elevation between larger sulci; ambit concave, bounded by sharp 
margins ; aperture one-half longer than wide, with sides indented ; siphunculus situate opposite inner indenta¬ 
tion; septa many, with straight margins. 
8. N. Polygonalis. {Mann-cornered Nautilus.) Spheroidal, umbilicate; inner volutions wholly concealed ; 
sides compressed, smooth; umbilicus minute; aperture large, arcuated, its reflected extremities nearly conceal¬ 
ing umbilicus; siphunculus situate near outer edge of septum and composed of many straight tubes, penetrating 
septa ; septa distant, concave; thickness equal to two-thirds diameter. 
9. N. Biangulatus. {Two-angled Nautilus.) Discoidal; sub-globose; umbilicate; inner volutions half- 
exposed ; ambit rounded ; umbilicus large, deep, in form of reversed cone, with margins acutely carinated; 
aperture elliptical, obtuse at extremities; width equal to two-tlnrds diameter of disc. 
10. N. Hexagonus. {Six-angled Nautilus.) Cylindrical, short, umbilicate; inner volutions wholly con¬ 
cealed ; sides depressed, with conical angle near centre; ambit broad, straight; umbilicus small; aperture 
sagittate, truncated on outer extremity; siphunculus situate near inner edge of septum ; septa numerous, 
slightly curved. 
11. N. Globatus. {Globular Nautilus.) Sub-globose, umbilicate; inner volution almost wholly concealed ; 
sides smooth ; ambit flattened ; umbilicus small, deep, with sub-carinated margin; aperture large, wide, 
arcuated, with deep, wide sinus on back; thickness and diameter equal; septa many. Greatest diameter 6 in. 
12. N. Woodwardii. (WoodwanV s Nautilus.) Discoidal, with four or five wholly exposed volutions; sides 
of volutions angular ; surface with many concentric series of granulated lines; ambit rounded, with linear 
sulcus at centre; aperture obovate with angulated sides. 
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