PLATE XX.* 
1. Ammonites Allasii. (Alias's Ammonite.) Six rounded volutions, wholly exposed, and 
crossed by many transverse, equidistant, bent ribs. 
2. A. Furcatus. (Forked Ammonite.) Discoidal; sides and front flat ; inner volutions partly 
visible; aperture, with square front, rather oblong, deeply impressed by preceding volution; lateral 
angles truncate; ribs rather distant, thick, curved, many forked, and passing at right angles over 
front. 
3. A. Cristatus. (Crested Ammonite.) Rather compressed; with sharp, smooth keel; trans¬ 
verse ribs very irregular, some furcated and bent, some single, and others only extending over half 
volution. 
4. A. Calcar. (Spur Ammonite.) Discoidal; inner volutions wholly concealed; sides with 
irregular, alternately long and short, curved ribs and central series of round tubercles; back with 
double keel of close-set, sharp tubercles. 
5. 6. A. CrenatllS. (Crenated Ammonite.) Sides flattened; inner volutions much exposed, 
with nearly smooth, rounded margins; those of outer volutions crenated on each side of concave 
space over sipliuncellus. 
7. A. Cookii. (Cook's Ammonite.) Aperture oblong; volutions moderately inflated, rapidly 
increasing, inner ones more than half concealed; surface smooth, with narrow ribs, every alternate 
one forked ; ambit slightly flattened, with ribs passing over it; four volutions. 
8. A. Fimbriatus. (Fringed Ammonite.) V. PI. XX., fig. 12. 
9. 10. A. Cireularis. (Circular Ammonite.) Aperture circular; volutions just touching each 
other; nearly close, acute, circular; surface sometimes with transverse ridges. 
11,12,13. A. Dissimilis. (Dissimilar Ammonite.) Inflated; with rapidly increasing volutions, 
smaller ones wholly concealed ; largely umbilicate; crossed by broad sulci and flattened, curved ribs 
over thick back, with a few, elongated nodules on inner edges of volutions; aperture wide, sub- 
orbicular. 
14,15. A. Rotifer. (Wheel-like Ammonite). Discoidal; with five rounded, smooth volutions, 
one-third concealed ; with series of close-set ribs over rounded back to nearly middle of volutions 
on either side, where they meet wide-set, thick ribs from inner margins ; aperture nearly orbicular. 
16. A. Mulgravius. (Mulgrave’s Ammonite.) Discoidal, umbilicate; volutions more than half 
concealed; internal sides bevelled inwards, inner edges defined by perpendicular, flat margin ; back 
narrow, with central, smooth, narrow, low keel; surface, with scries of falcated sulci, from inner 
edges in fine lines. 
17, 18. A. Symmetricus. (Si/mmetrical Ammonite). Aperture almost square ; with rounded, 
notched keel ; ribs obtuse, tumid as they approach hollow, where keel is immersed ; sides of volutions 
flattened. 
19. A. Gamma. (Gamma Ammonite.) 
20,21. A. Trisercalis. (Three-Tub&rcled Ammonite.) Discoidal, umbilicate; with flattened 
margin; sides with series of straight, transverse ribs, each with rounded, obtuse tubercle on outer 
end; marginal portion of rays, with three rows of tubercular protuberance, and about ten tubercles 
around umbilicus ; aperture nearly square. 
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