Ammonites. 
MOLLUSCA. 
9 
which they are abruptly reflected, and imperceptibly disap¬ 
pear ; back provided with an obtusely rounded carina, on 
each side of which is a furrow. Diameter varying from a 
foot to twenty-one inches. 
First discovered by that zealous geologist, Professor Buck- 
land, in the Blue Lias at Bath and its vicinity. 
29* A. VAHIANS_ The Variable Ammonite, pi. VII. figs. 
3, 5, and 8. 
Sowerby, Min. Conch. II. p. 169? pi* 176* Mantell, Geo¬ 
logy of Sussex, p. 115, pi. 21, figs. 2, 5, and 7* 
Discoidal, compressed, rather thick, subumbilicate, carinated; 
with three or four half inserted volutions; furnished with 
transverse, bifurcated, undulated ribs, studded with from six 
to eight rows of somewhat obtuse tubercles ; carina acute, 
entire ; aperture sagittate ; siphuncle supposed to be external. 
The umbilicus is shallow, and the sides smooth, bor¬ 
dered by a row of small tubercles, from which the ribs ema¬ 
nate, and proceeding obliquely across one-fourth of the 
volutions, rise in the form of’ tubercles, and then diverge into 
two branches, all of which terminate in a tubercle on the 
exterior margin ; the keel is smooth, prominent, and acute ; 
each margin furnished with a series of opposite tubercles. 
This species is one of the most proteiform of the Ammo¬ 
nites, and subject to great variety in the form, disposition, 
and number of the tubercles and cost* ; but its acute, entire 
carina, in connection with the tubercular, bifurcated ribs, at 
once distinguish it. 
In size this fossil varies from an inch to six inches in 
diameter, and is not unfrequently compressed into an ellipsis 
or cordiform shape. No specimen has been yet found with 
more than four volutions. 
Mr Mantell says, “ In a suite of fifty specimens, in which 
every individual presented some peculiarity, three principal 
varieties were observable, each passing insensibly into the 
other.” 
Variety 1. subplana , pi. VII. fig. 8. 
Mantell, Geology of Sussex, pi. 21, fig. 2. 
“ The volutions depressed, radii linear, inner row of 
jtubercles obscure, external margin crenated, keel but slightly 
elevated, aperture sagittate. 
Some specimens of this variety are nearly smooth, and the 
keel so much compressed, that without the aid of numerous 
examples, their relation to the tubercular variety could not 
have been ascertained. 
Variety 2. intermedia . 
Mantell, Geology of Sussex, pi. 21, fig. 7, 8. 
The volutions in this variety are rather depressed, the ribs 
broad and well defined, the tubercles small, and distinctly 
marked, the external margin tuberculated, the keel prominent, 
and the aperture sagittate. 
This is the prevailing form of the species, and holds 
an intermediate rank between the smooth and tubercular 
varieties. 
Variety 3. luberculata. 
Sowerby, Min. Conch, pi. 176, figs. 1, 2, 3, 4, 6. 
Volutions subrotund ; ribs short, thick, nodulous ; tubercles 
elongated, very prominent ; carina acute ; aperture somewhat 
rounded in form. 
“ A very beautiful variety, distinguished by its projecting 
tubercles, of which Mr Sowerby’s fig. 1, affords an excellent 
example. The inner rows of tubercles are almost effaced, 
but the marginal and intermediate sets are strongly relieved, 
and in some examples become spinous. From the thickness 
of the volutions, the aperture is obovate.” 
This fossil is plentiful at Middleham, Hamsey, and 
Stoneham, in Sussex, and also in the upper Green-sand of 
Wiltshire. The Gray Chalk Marl is well marked by this 
shell, as it prevails abundantly through it. Mr Mantell 
mentions that a few examples have been found in the lower 
or flinty Chalks. 
30. A. Duncani.— Duncan’s Ammonite, pi. VII. fig. 4. 
Sowerby, Min. Conch. II. p. 129, pi* 157* 
Compressed, inner volutions exposed about a third, with a 
few tubercles upon their sides, the whole shell beset with 
numerous, undulating, narrow, oblique ribs, many of which 
are irregularly furcated, somewhat obscure on the middle of 
the sides, and terminated on their outer extremities by 
elongated tubercles on the terminal half of the exterior 
volution, but button-shaped on the other half, these last, in 
many instances, extend over two of the cost*; there is also 
a row of tubercles on the sides of the latter half, towards the 
centre of the volution ; ambit depressed, bounded by two 
rows of fibuliforni tubercles, which are a continuation of the 
ribs ; aperture ovato-sagittate, and equal to about half of the 
greatest diameter of the shell. 
The sinuated edges of the septa are sharp and distinctly 
marked. 
Discovered in the Fen Clay at St Neotts, Huntingdon¬ 
shire, by John and Philip Duncan, Esqrs. in honour ot whom 
it was named by Sowerby. 
31. A. Co NY be A hi. —Conybear’s Ammonite, pi. VII. fig. 6. 
A. Conybearu Sowerby, Min. Conch. II. p. 70> pi* L31 '•> 
Phillip’s Geology of Yorkshire, II. p. 164, pi. 13, fig. 5. 
Compressed, with a large, greatly produced, entire keel, on 
each side of which is a concave groove ; volutions eight or 
nine, usually continuing very perfect to the centre, crossed 
by numerous rather obtuse ribs, which are most prominent 
in tiie centre of the volutions, and are much depressed at the 
inner sides, a little stronger next the ambit; inner sides of 
the volutions somewhat flattened, and slightly angular; 
aperture oblong-ovate. Varying in size from two to eighteen 
inches. 
Found in the Lias at Bath, and in the middle of England, 
where it is not uncommon, and in the western islands ol 
Scotland. 
32. A. plan 1 costatus. —The Flat-ribbed Ammonite, pi. 
VII. fig. 7* 
A. 2 >lanicosta. Sowerby, Min. Conch. I. p. 167> pi* 7*3. 
Compressed, with six or eight exposed volutions, crossed 
by numerous obtuse, nearly straight ribs, which widen as they 
approach the back, and are depressed near the ambit, inclining 
towards the aperture ; aperture circular, slightly indented by 
the volutions. 
Found in the indurated marly limestone, called Marston 
Stone, which occurs at Marston Magna, near Ilchester, at 
Yeovil and Evcrshot, Somersetshire. This stone is frequently 
cut into large slabs for table-tops, &c. and when polished has 
a beautiful effect from the irregular order in which these 
numerous specimens of Ammonites planicostatus present 
themselves. The limestone is of a dark gray colour, and the 
Ammonites are dark brown, or different shades of buff, and 
sometimes exhibiting a splendid iridescent lustre. The 
divisions of the chambers being filled with crystallized car¬ 
bonate of iron, adds a beautiful variety to the tints, 'ihis 
c 
