Ammonites. 
MOLLUSCA. 
b 
Genus IV.—AMMONITES.— Lamarck . 
Shell discoid, multilocular; volutions contiguous, all 
visible; inner partitions articulated by sinuous sutures; 
septa transverse, lobed at the circumference and imper¬ 
forated at the disc, but perforated by a single tube situate 
near the margin. 
In the extensive genus Ammonites the situation of the 
siphuncle is always upon the ambit or dorsal margin of 
the transverse plates, as shewn in the perpendicular 
section of Ammonites obtusus , pi. IV. fig. 8 . It is repre¬ 
sented in black, and marked by the letters c , r/, e,/ < 7 , h. 
It is conducted through the plates by a ring, projecting 
outwards, and may be traced passing through the whole 
transverse plates of the above figure. The body of the 
animal has occupied that portion of the shell from a to b. 
The Ammonites occur in all formations from the 
transition strata, and disappear with the termination of 
the Chalk. 
1 . A. Stellaius—T he Star Ammonite, pi. IV. fig. 2. 
Sovverby, Min. Couch. I. p. 211, pi. 93. 
Involute, with four somewhat depressed volutions; ob¬ 
tusely eariuated, on each side of which is a rounded furrow; 
inner volutions about two-thirds visible, with their sides flat¬ 
tened, producing a pentagonal or stellated appearance ; with 
numerous straight, moderately raised radiating ribs; the 
whole surface of the shell covered with obscure, rather remote 
decussating striae; sinuous margin of each septum crossed by 
two costa?; the septa situated at each fourth rib; siphuncle 
placed in the keel; aperture quadrangular, rather longer than 
wide, its length being two-fifths the diameter of the shell. 
Greatest diameter four aud a half inches. 
This species is common at Lyme Regis, Dorsetshire. 
2. A. Lewesiensis. —The Lewes Ammonite, pi. IV. fig. 3. 
Man tell, Geology of Sussex, p. 199? ph 22, fig. 2 . 
Depressed ; three or four wide, flattened volutions, almost 
entirely concealed, and with four or five obscure, radiating 
ridges ; septa sinuous, very numerous, and the surface usually 
covered with thin, foliaceous impressions; external volution 
equal to four-sevenths of the diameter of the shell; umbilicus 
minute ; carene very narrow, rounded ; aperture sagittate. 
Largest diameter usually about fourteen inches; but speci¬ 
mens have been found eighteen inches in diameter. Width 
of the outer volution commonly nine inches; greatest thick¬ 
ness five inches, and at the external edge one and a half inch. 
The greatest thickness of this shell is at the inner margin, 
from whence the volutions gradually taper to the keel. The 
outer volution increases abruptly, and is nearly equal to half 
the diameter of the shell. Mantell says, “ This ammonite 
may be readily distinguished in a suit of specimens, although 
its characters are rather of a negative description. In its 
general form it resembles Ammonites complanatus , ( Gray 
Marie Fossils , No. 34 ;) but the umbilicus is larger, the 
carene less acute, and the surface exhibits no traces of strim 
or pliem. In the larger specimens the volutions appear to be 
wholly inserted ; but probably, in more perfect examples, 
their inner margin is exposed.” 
Found in the Lower chalk near Lewes. 
3. A. binus. —The Coupled Ammonite, pi. VII. fig. 11. 
Sowerby, Min. Conch. I. p. 208, pi. 92, fig. 3. 
Involute, depressed ; volutions four; the inner ones about 
two-thirds exposed; ribs radiating in pairs, emanating from 
round tubercles, which are situate near the inner margin of 
each volution, turgid, and then bent up towards the front, 
where they become obsolete; keel small, entire; aperture 
oblong, rectangular, one-third the diameter of the shell, and 
a little more than one-sixth wide, with somewhat rounded 
angles. In some specimens a single rib occurs between the 
pairs. 
Found at Bramerton, Norfolk. 
4. A. striatus. —The Striated Ammonite, pi. IV. fig. 6 . 
Sowerby, Min. Conch. I. p. 115, pi. 53, fig. 1 . Goniatites 
striatus , Phillip’s Geology of Yorkshire, II. p. 233, pi. 19, 
figs. 1—3. 
Discoidal, very gibbose, its thickness being more than half 
its diameter; inner volutions entirely concealed ; outer sur¬ 
face obscurely undulated transversely, and covered with nu¬ 
merous fine, very regular, close, concentric striae, the undula¬ 
tions traverse the surface in very regular semicircular 
incurvations with the acute terminations meeting in points 
upwards, and gradually passing into straighter lines on the 
sides; aperture semicircular, with nearly parallel margins; 
septa zigzag, rather remote, with four large, somewhat angular 
folds ; siphuncle placed at the external margin of the septum, 
where it is provided with a slight notch ; shell very thin. The 
zigzag divisions arc well marked, without passing into foliated 
sutures as is usual in the genus Ammonites . Dorsal lobe bifid ; 
dorsal sinus and first lateral lobe acute, and twice the length 
of the dorsal lobe; second lateral lobe obtusely rounded, 
shorter than the first; marginal sinus angular ; siphuncle not 
continuous, but passing rectally from the septal plate for a 
short distance. 
1 he ribs and striae of the external shell are strengthened by 
the repeated intersections of the subjacent edges of the trans¬ 
verse plates. 
Found in Pools-hole in the Peak of Derbyshire, and in the 
transition slate of Filiagh, near South-molton, Devonshire ; 
Bolland, Flasby; and also in Coal shale of Lough Allen in 
Connaught, Ireland. 
5. A. spheric us.—The Spherical Ammonite, pi. IV. fig. 7. 
Martin, Petrefactions of Derbyshire, pi. 7, figs. 3, 4, and 5. 
Sowerby, Alin. Conch. I. p. 116, pi. 53, fig. 2 . Goniatites 
spkaricus , Phillip’s Geology of Yorkshire, II. p. 234, pi. 19, 
figs. 4, 5, 6 . 
Orbicular, diameter and thickness nearly equal ; inner 
volutions entirely concealed ; outer surface with very fine 
spiral stria?; internal ridges variable; septa with four broad, 
angular folds, as in the preceding species ; but the lateral lobe 
is less acute, or even rounded; aperture a little contracted. 
Found in the Limestone of Derbyshire, and at Bolland ; 
Isle of Man ; and In the county of Kildare, Ireland. 
This grows to double the size of A. striatus . 
6 . A. Mantelli. —Mautell’s Ammonite, pi. IV. fig. 4, 9 . 
Sowerby, Min. Conch. I. p. 119, pi. 55. Mantell, Geology 
of Sussex, p. 113, pi. 21, fig. 9 , and pi. 22, fig. I. 
Discoidal, depressed, subumbilicate; volutions three or 
four, subrotund, about two-thirds concealed, margin trigonal; 
with numerous transverse tuberculate ribs, which alternately 
reach entirely round the volutions, the shorter ones extending 
about two-thirds across the volutions; with from two to eight 
rows of tubercles; ambit flattish, provided with two rows of 
marginal tubercles ; external edges of the septa with five 
B 
