PLATE XX. 
1. Ammonites Baltiatus. (Belted Ammonite.) Discoidal; volutions wholly exposed; crossed 
by numerous, strong, elevated ribs, from inner margins of volutions, and passing over rounded ambit. 
2. 9. A. Decipiens. (Deceitful Ammonite.) Discoidal, thick, with five or six exposed volutions, 
crossed by large, elevated ribs, rising near inner margin, and crossing sides, become obscure towards 
rounded ambit; aperture oblong. 
3. A. Ovatus. ( Ovate Ammonite.) Lenticular, umbilicate; inner volutions wholly concealed, 
their margins alone visible within small, deep umbilicus; sides thick internally, sloping towards 
thin, flattened back, crossed by many undulating depressed ribs from internal margin. 
4. A. Venustlis. (Graceful Ammonite.) Discoidal, thick; with three or four volutions, inner 
ones half concealed; sides crossed by many, straight, elevated ribs, met at centre by many, strong, 
rounded, curved ribs, passing over ambit and meeting with those on opposite side, producing crenated 
margin at back ; aperture oblong. 
5. A. Concinnus. (Cornell/ Ammonite.) Discoidal, thick; with four volutions, inner ones half- 
inserted, with series of strong, remote ribs from inner margins of volutions, met at centre by many 
acute elevated ribs, passing over rounded ambit; aperture roundish. 
0. A. Minimus. (Smallest Ammonite.) Discoidal, thick; with three volutions; inner ones 
two-thirds inserted, outer ones increasing rapidly; sides crossed by many, curved ribs, from inner 
margins, and passing over rounded ambit; aperture oblong. 
7. A. Curvinoides. (Curved-ribbed Ammonite.) Volutions flattened, with remote, elevated, 
waved ribs from inner margins of volutions, and near ambit, suddenly and acutely curving towards 
aperture; between these, several depressed and nearly obsolete ribs; ambit thinnisli. 
8. A. Varicostatus. (Variously-ribbed Ammonite,) Discoidal; with six, two-tlxirds exposed, 
rounded volutions ; inner ones crossed by many, narrow, greatly elevated, close-set. ribs, bifurcated on 
rounded ambit; near termination of outer volution, ribs become large, broad, distant, slightly curved, 
without dorsal bifurcation; aperture oblong-ovate. Greatest diameter, 9 in. 
10. A. Rotundus. (Round Ammonite.) Discoidal; volutions exposed ; sides crossed by many 
thick ribs, bifurcating on passing over back; aperture orbicular, interrupted by small sinus from 
insertion of preceding volution. 
11. A. Heterophyllus. (Reversely-foliated Ammonite.) V. PI. XVI., fig. 3. 
12. A. FimbriatliS. (Fringed Ammonite.) Discoidal; with cylindrical volutions, inner ones 
wholly exposed, crossed by many lines of growth, with fimbriated margins; aperture orbicular, with 
undulating frill. 
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