MOLLUSCA. 
Ammonites. 
In the young shell, the septa are much less sinuated than 
in the adult, which is finely exemplified by a specimen in the 
possession of the Marchioness of Bath. 
Found in the Lias at Whitby, Yorkshire. 
Professor Buckland makes the following observations on 
this interesting and curious species ; referring to its foliage, 
he says, “ Its laws of dentation are the same as in other Am¬ 
monites, but the ascending secondary saddles, which in all 
Ammonites are round, are in this species larger than ordinary, 
and catch attention more than the descending points of the 
lobes” 
The figures of the edge of one transverse plate are repeated 
in each successive plate. The animal, as it enlarged its shell, 
thus leaving behind it a new chamber, more capacious than 
the last, so that the edges of the plates never interfere or 
become entangled. 
Although the pattern on this Ammonite is apparently so 
complicated, the number of transverse plates is but sixteen 
in one revolution of the shell ; in this, as in almost all other 
cases, the extreme beauty and elegance of the foliations result 
from the repetition, at regular intervals, of one symmetrical 
system of forms, namely, those presented by the external 
margin of a single transverse plate. No trace of these folia¬ 
tions is visible on the outer surface of the external shell, as 
will be seen by our representation, pi. XVI. fig. 3. 
The elevations and depressions on the ambit of this species, 
strongly illustrate the theory of Von Buch, respecting the 
use of the lobes and saddles, formed by the undulations of 
its outer margin.* 
118. A. fissicostatus. — The Cleft-Ribbed Ammonite, 
pi. XVI. fig. 4. 
Ammonites Jissicostalus. Phillips, Geology of Yorkshire, 
I. p. 123, pi. 2, fig. 4f). De la Beche, Geo. Manuel, p. 294. 
Discoidal, with five or six two-thirds exposed, somewhat 
depressed volutions, crossed by numerous strong, slightly 
undulated ribs, which become furcate about the centre, and 
passing over the rounded ambit, meet and join with the cleft 
ribs of the opposite side; aperture subovate, its width being 
about five-sixths of its length. 
Found in the Speeton Clay, Yorkshire. 
119. A. gemmatus. — The Gemmed Ammonite, pi. XVI. 
fig. 5. 
Ammonites gemmatus . Phillips, Geology of Yorkshire, I. 
p. 141, pi. G, fig. I?. He la Beche, Geo. Manuel, p. 371. 
Discoidal, with four or five ventricose volutions, the inner 
ones half exposed, and much thinner as they approach the 
centre ; inner margin of the volutions slightly flattened, from 
which emanate numerous, somewhat distant, strong, elevated, 
nearly straight ribs, which, on reaching the centre of the 
sides, are united to large, rounded, obtuse tubercles ; from 
which the ribs are continued diagonally, and terminate in 
large, slightly conical tubercles, on both sides of the broad and 
flattened ambit; between each of the exterior half of these 
ribs and the back, are two or three smaller costa?, which also 
terminate on the ambit. Greatest diameter six inches. 
Discovered in the Kelloways Rock, Yorkshire. 
120. A. subl^evis.—T he Half Smooth Ammonite, pi. XVI. 
fig. 6. 
Ammonites subtree is. Sowerby, Min. Conch. I. p. 117, pi. 
54. Phillips, Geology of Yorkshire, I. p. 141, pi. G, fig. 22. 
* See our Elements of Fossil Conchology, where this theory is fully 
explained. J 
De la Beche, Geo. Manuel, p. 370. Orbulites Icevis, Lamarck, 
54. Orbulita modiolaris , Fleming, Brit. An. p. 248. Nautilus, 
See. Luidii Lithoph . Brit. Tab. G, fig. 292. 
Orbicular, umbilicate; inner volutions all enveloped in the 
exterior one, and their crenated edges visible only within the 
deep conical umbilicus, which is undulated internally, and 
provided with a sharp angular edge ; aperture semicircular, 
very wide, and truncated at the sides ; whole outer surface 
smooth, and devoid of inequalities ; septa numerous, with fine 
prominent undulations, which are divided into many lesser 
and rounded foliations, their margins resembling pinnate 
leaves ; siphunculus close to the outer margin of the ambit; 
diameter, five inches, umbilicus, an inch and a quarter in 
width. 
This species varies much in its young and adult state. 
When young, it is considerably compressed, and its surface 
provided with slight furcate undulations. I 11 the infant state, 
its width is only half its length, and exhibits, on its surface, 
transverse ribs ; these become less acute, and assume a more 
rounded and depressed form, as the shell progresses, and 
consist of longer and shorter ones, some of which are 
bifurcate. When the shell has reached three inches, these 
become less conspicuous, and when it has grown to four or 
five inches in diameter, they are entirely obsolete.* 
Found in the Coral, Oolite, and Kelloways Rock, York¬ 
shire, and Middle and South of England ; the Fuller’s Earth 
of Bath ; also in the Oxford Clay of Begginger, Schafen- 
hausen, according to V 011 Bucli ; and De la Beche says it is 
found in the same kind of strata, Normandy. 
121. A. flbx 1 costatus. —The Bent-Ribbed Ammonite, pi. 
XVI. fig. 7. 
Ammonites Jicxicostatus. Phillips, Geology of Yorkshire, 
I. p. 142, pi. 6, fig. 20. De la Beche, Geo. Manuel, p. 371. 
Discoidal, compressed ; with four or five two-thirds ex¬ 
posed volutions, slightly raised in the centre of the sides, and 
slanting off to the margins, crossed by numerous, elevated, 
bent ribs, which become furcate in the middle, and passing 
over the somewhat thin and flattened ambit, meet with and 
join the furcations on the opposite side ; the outer volutions 
increase rather rapidly ; aperture oblong-ovate. 
Found in the Kelloways Rock at Hackness, Yorkshire. 
122. A. ckassus. —The Thick Ammonite, pi. XVI. fig. 8. 
Ammonites crassus. Phillips, Geology of Yorkshire, I. p. 
163, pi. 12, fig. 15. De la Beche, Geo. Manuel, p. 371. 
Young and Bird, Geo. Yorkshire, p. 
Discoidal, with five thick half inserted volutions, somewhat 
flattened on their sides, crossed by numerous, straight, pro¬ 
minent, rounded ribs, which, on reaching the centre, are pro¬ 
vided with a round, elevated tubercle, from which the ribs 
split into two or three divergent smaller cost#, and passing 
over the thick rounded ambit, unite with those of the opposite 
side ; aperture, subcordiform. 
This species has some affinity to Ammonites JBroduei, pi. 
X. fig. 13, but may at once be distinguished from it by its 
inferior size, its greater proportional thickness, in having 
fewer volutions, and the central ones being thicker in all stages 
of growth, and in the external minor ribs being fewer and 
less elevated. 
Found in the upper Shale, Lias formation, of Yorkshire. 
123. A. flicatilis. — The Plaited Ammonite, pi. XVI. 
fig. 9. 
* See our Elements of Fossil Conchology. 
