Ammonites. 
MOLLUSCA. 
Depressed, with usually six volutions; the surface covered 
with numerous, sometimes furcated well rounded ribs, and 
intermediate shorter ones extending to half the breadth of 
the volutions; inner volutions exposed ; sides somewhat 
straitened ; aperture obovate ; septa numerous, with greatly 
sinuated margins. Thickness about equal to one-fourth of 
its diameter. 
This species is the largest of the genus. There is a spe¬ 
cimen in the Museum of the Jardin des Plantes, Paris, four 
feet in diameter. One was said to have been broken at 
Chicksgrove quarry, near Ilindon, Wiltshire, in a compact 
sandy limestone, which was as large as the hinder-wheel of a 
carriage. Specimens two feet in diameter are not uncommon. 
It is found, besides the above locality, at Peerbeck Isle, 
Dorsetshire; Marleborough Downs, in the Chalk near Mar¬ 
gate ; and at Fonthill. 
14. A. ELLiPTicus.—The Oval Ammonite, pi. V. fig. 7- 
Sowerby, Min. Conch. I. p 209, pi. 92, fig. 4. 
Depressed, with a sharp keel; the interior volutions two- 
thirds exposed ; ribs few, distant, broad, flat, agreeing in 
number with the septa, and slightly curved, somewhat obso- 
latc near the margin ; aperture oblongly elliptical. 
Found in the Marley clay at Charmouth. 
15. A. coknuoides.—T he Little-horn Ammonite, pi. V. 
fig. 8. 
Involute, depressed, with a broad, flattened keel ; the 
whole surface covered by rather prominent, gently bending, 
distinct ribs, extending from the internal margin to the 
carina, thickening outwards ; inner volutions considerably 
exposed ; aperture subcordate. Diameter an inch and a 
quarter; thickness three-eighths. 
Found at Whitby. 
16. A. triplicates. —The Three Pleated Ammonite, pi. 
V. fig. 9. 
Ammonites triplicatus .—Sowerby, Min. Conch. III. pi. 292, 
and 293, fig. 4. 
Discoid, with six exposed volutions, the two external ones 
separated by a depression or flattened spiral groove ; the 
whole external surface covered by strong, equidistant, regular, 
slightly bent ribs, extending from the interior side to nearly 
the external side, where they cease, the spaces between them 
being greater than the thickness of the ribs; aperture sub- 
cordate. Diameter eight inches, and equal to four times its 
thickness. 
Found near Malton, Yorkshire, and in the Suffolk Clay. 
17. A. biplex. — The Two-Pleated Ammonite, pi. V. 
fig. 10. 
Sowerby, Min. Conch. III. p. 167, pi. 293, fig- L 2. 
Discoid, with six exposed volutions, all separated by a 
depression or flattened groove ; furnished with large equi¬ 
distant, regular elevated ribs, extending in a straight line 
from the margin of the separating groove to two-thirds 
across the volutions, where they arc furcated, and pass 
over the dorsal margin, which is rounded ; aperture oblong, 
subcordate. Diameter eight inches ; thickness a fourth of 
its diameter. 
Found in the Suffolk Clay, and also in the London Clay. 
18. A. Bkongniarti. — Brongniarte’s Ammonite, pi. VI. 
fig. 1 . 
Sowerby, Min. Conch. II. p. 190, pi. A. fig. 2. 
Gibbous ; thickness about two-thirds its diameter; with a 
minute umbilicus ; round within, but externally oblong, pro¬ 
duced by the line of last volution, being straight for a little 
distance, from whence it makes a sudden turn towards the 
aperture ; inner volutions concealed ; whole surface covered 
with close, undulating, very regular, rather depressed, fur¬ 
cated radii ; aperture placed transversely, provided with a 
thick inflected lip. 
Found at Yeovil and in the Marley Limestone, Normandy. 
19- A. Calloviensis.—T he Kelloways Ammonite, pi. VI. 
fig. 2. 
Sowerby, Min. Conch. II. p. 3, pi. 104, fig. 1. 
Involute, subumbilicate, with five volutions, three-fourths 
concealed; front, or ambit, depressed ; with very numerous, 
small, bent, radiating ribs, arranged in sets, with a stronger 
one reaching across the volution, and from two to five 
shorter ones, alternating with a longer rib over the whole 
surface ; these are somewhat obscure in the external volutions 
of adult shells, in which the aperture is deltoidal, with 
truncated angles, but obicular in young specimens ; siphuncle 
placed near the upper edge. Greatest diameter three 
inches. 
Found in the Shell-Limestone at Kelloway’s Bridge. 
The form of the volutions in this species is much influenced 
by age. When young, they are somewhat rounded, with 
numerous sharp ribs arranged in sets ; a series of produced 
ones, between every two of which are placed from two to 
five shorter and more depressed cost®, reaching about two- 
thirds across the volutions ; the whole ribs passing over the 
flattened ambit. The outer volutions of adult specimens are 
triangular, the two inner angles being truncated, producing 
an umbilicated aspect; the surface with large undulations, 
wrinkled near the ambit, and provided with numerous irre¬ 
gular stria* in place of ribs ; they differ also in the inner 
surface of the outer volutions being destitute of stria*, and in 
losing the ribs sooner. The shell is thick and is frequently 
well preserved. 
20. A. Gervilii.—D c Gerville’s Ammonite, pi. VI. fig. 3. 
Sowerby, Min. Conch. II. p. 189, pi. A, fig. 3. 
Gibbous, largely umbilicate, exposing the ribbed margins 
of the volutions; thickness somewhat more than half the 
diameter; with sharp, numerous, close, very regular, bent, 
furcated ribs, continuing so to near the completion of the 
last volutions, when they are supplanted by two or three 
irregular undulations ; inner volutions but slightly exposed; 
aperture transversely oblong, and excavated ; lip sharp on 
the edge, and arched. 
Found in Marley Limestone. 
21. A. obtusus. —The Obtuse Ammonite, pi. VI. fig. 4. 
and pi. IV. fig. 8. 
Sowerby, Min. Conch. II. p. 151, pi. 167 ; Buckland’s 
Bridgewater Treatise, I. p. 347, and II. p. 58, pi. 35, 36. 
Discoidal, with an obtusely rounded, considerably elevated 
keel, with a slight furrow on each side ; furnished with six 
volutions, the inner ones wholly exposed, covered with large, 
curved, remote, slightly elevated, strong ribs, equal in 
number to the septa; each crossing the inner lobes of a 
septum; somewhat sharp in the middle ; aperture oblong, 
longer than wide, about equal to one-third the diameter of 
the shell. Largest diameter five inches and a half. 
Found in the Lias at Lyme Regis, Dorsetshire. 
Sowerby mentions a specimen from which he made his 
drawing, sent to him by Miss Philpot of Linlcy, “ which, 
from the high polisli and rich colour of the crystallized 
