PLATE XVII. 
1. Ammonites Johnstoni. (Johnston's Ammonite.) Discoidal; with six to nine greatly-compressed, one- 
third inserted volutions ; crossed by central series of many short, straight, elevated ribs or elongated tubercles ; 
inside of volutions plain ; ambit plain ; central volutions without ribs. 
2. A. Rotiformis. (Wheel-shaped Ammonite.) Depressed, carinated ; sides somewhat concave ; with six to 
eight wholly exposed volutions ; crossed by many distant, strong, slightly bent, elevated ribs, each with oblong, 
obtuse tubercle at outer extremity ; ambit flat; keel slightly sunk, with sulcus on both sides ; aperture almost 
square, only one-sixth diameter of shell. Greatest diameter, 7 in. 
3. A. Bifrons. (Two-fronted Ammonite.) Discoidal, sub-umbilicate; with three or four convex volutions, 
inner ones one-third concealed ; series of remote, rounded, prominent, nearly straight ribs from inner margin of 
volutions, and on reaching two-thirds across sides, terminate and meet many smaller ribs, passing over rounded 
ambit, producing crenulated profile. 
4. A. Longispinus. (Long-spined Ammonite.) Discoidal, thick; with three or four plain, half-inserted 
volutions; sides with two concentric series of spiniform tubercles; ambit thick, smooth, gently rounded; 
aperture orbicular, deeply indented by contiguous volution ; greatest diameter rather more than three-fifths 
diameter of disc ; external surface pearlaceous. 
5. A. Contractus. (Contracted Ammonite.) Subglobose, deeply umbilicate; inner volutions nearly con¬ 
cealed ; sides contracted; around umbilicus series of large, rather acute tubercles, whence numerous, smooth, 
rounded ribs, furcating into sets of three or four, and passing over greatly-rounded ambits join those on 
opposite side ; aperture oblong, arcuated. 
6. A. Turneri. (Turner's Ammonite.) Depressed, carinated ; with five volutions, inner ones almost entirely 
exposed; sides flattened, with many equal ribs; carina only moderately elevated, with sulcus on each side; 
aperture oblong, quadrangular, about one-third diameter of disc. 
7. A. Lseviusculus. (Smoothish Ammonite.) Discoidal, carinated, umbilicate ; with four or five volutions, 
inner ones partly exposed within small, shallow umbilicus; outer volution very large, its sides rather convex, 
with slightly elevated, smooth, waved, alternately long and short ribs ; ambit obtuse ; carina large, prominent; 
aperture sagittate, occupying half diameter of disc ; size of umbilicus, one-third of remaining half. 
8. A. Striatulus. (Minutely-striated Ammonite.) Discoidal, carinated; with six convex, wholly-exposed 
volutions; sides, with many slender, doubly-undulated ribs; whole surface, with minute stria}, lying parallel to 
ribs; carina slightly produced ; aperture elliptical, its length about one-fourth diameter of disc. 
9. A. Banksii. ( Banks 9 Ammonite.) Discoidal, umbilicate, very thick ; with five or six rounded volutions, 
margins well relieved ; sides concave, w r ith row of ten or eleven large, round, obtuse tubercles; back slightly 
convex, with series of oblique, fluted grooves towards aperture ; inner volutions narrow, outer one very thick, 
equal to remainder of disc; aperture transverse, twice as broad as long. 
10. A. Rusticus. (Rude Ammonite.) Depressed; with about three gibbous volutions; with row of conical, 
obtuse tubercles, and two rows of slightly elevated ones, round broad and flattened ambit on both sides; aper¬ 
ture wider than long, its inner side concave, shorter than other angles. 
( 18 ) 
