PLATE XXIY. 
1. Nautilus Centralis. ( Central-Siphuncled Nautilus.) Involute, umbilicate; inner volutions 
wholly concealed; aperture transversely elongate, bluntly lunate; sipliunculus central; umbilicus 
large, situate behind projecting sides of aperture; sides smooth; septa entire, bending, concave. 
2. N. Costalis. (Bibbed Nautilus.) Spheroidal, umbilicate; inner volutions wholly concealed; 
sides with transverse flutings or ribs; ridges rising on margin of deep umbilicus, and ending in 
acute projections on ambit; with strong, elevated stria?, parallel to ridges; aperture lunate. 
3. N. Ziczac. (Zigzag Nautilus.) Involute; sides flattened ; inner volutions wholly concealed ; 
aperture bluntly triangular; sipliunculus situate nearest to inside ; septa concave, recurved at ends, 
producing zigzag appearance; thickness one-third diameter. 
4. N. Striatus. (Striated Nautilus .) Discoidal, gibbose; inner volutions few, wholly concealed; 
sides with very strong, elevated, many, irregular concentric strife; umbilicus large; ambit compressed; 
aperture orbicular, flattened on back, equal to half diameter of disc. Greatest diameter 8 in. 
5. N. Imperialis. (Imperial Nautilus.) Discoidal, umbilicate; inner volutions wholly con¬ 
cealed; sides gibbose, smooth, but striated concentrically in young state; aperture lunate, with 
blunted sides ; sipliunculus situate nearest to inside; septa entire, concave; truncated and somewhat 
recurved at ends; umbilicus rather large. 
6. N. Bilobatus. (Tivo-Lobed Nautilus.) Sub-globose, umbilicate ; inner volutions wholly con¬ 
cealed; sides smooth; back flattened; umbilicus minute, cylindrical; aperture transversely elongate, 
its width nearly four times its length; sipliunculus central, on each side of which septa very concave, 
forming two elliptical lobes; thickness and width about equal. 
7. N. Intermedius. (Intermediate Nautilus.) Globose, umbilicate; inner volutions half- 
exposed ; sides rather depressed, concentrically striated; ambit broad, flattened; aperture rather 
quadrangular; sipliunculus round, placed nearest external margin; septa many. 
8. N. Obesus. (Swollen Nautilus.) Gibbose, umbilicate; inner volutions wholly concealed; 
sides plain; ambit broad, flat; aperture very large, rather quadrangular ; its length about two-tliirds 
diameter of disc; sipliunculus oblong-ovate, transverse, central ; septa very numerous; umbilicus 
large. Greatest diameter 13 in. 
9. N. Complanatus. (Flattened Nautilus.) Discoidal, compressed, with four or five volutions; 
inner ones wholly exposed; their inner edges flattened, leaving concave surface; half of last volution 
without septa; sides flat, smooth; aperture lanceolate; its length nearly six times its width; near 
inner angle, edge of each septum with reversed sinus; ambit rounded. 
10. N. Tuberculatus. (Tuberculated Nautilus.) Discoidal, thick, largely umbilicate; inner 
volutions almost wholly exposed; outer volutions thick; umbilicus deep, shape of reversed cone; at 
one-third of breadth of volutions is a series of large, round tubercles; ambit rounded; aperture 
transversely elongate and somewhat bi-angular, its width being twice its length; septa slightly 
concave, with edges waved. 
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