CEPHALOPODA Am monites excavatus 
Sowerby, 1818. 
Original description 
(1818. — Sowerby, Min. Conch., Vol. Il, p. 5-6, PI. CV). 
« AMMONITES excavatus. TAB. CV. 
« SrEC. CHAR. Involute, lenticular, subumbilicate ; keel sharpish, crenu- 
lated ; whoris about six, those of the young shell exposed ; inner margin 
nearly right-angled; radii curved, obscure in full grown shells ; aperture 
in adult shells sagittate ; inner angles truncated. 
« À SLIGHT concavity separates the keel from the remaining uniformly convex sides ; the 
radiüi, which in the last-formed whorls of full-grown shells are little more than irregular 
striæ, are in the first whorls very prominent over the inner angles ; these angles in the 
young shell are rounéed, and have not the flat space which in the old shells forms a partial 
umbilieus. The length of the aperture is about half that of the diameter of the shell, and 
the width at the back about one-third. The chambers are rather distinct. 
« I had the pleasure of picking this up a few years ago on the productive hill of Shotover 
near Oxford, and as I saw no more specimens it may possibly be rare. The shell which is 
replaced by Carbonate of Lime is moderately thick, and appears very exact in most parts. 
The chambers are commonly hollow, and crystallized within; the thin septa are also dis- 
tinctly replaced by Carbonate of Lime. The finishing chamber, which would add a fourth 
to the size of the whole, and which I have separate, is filled with [6] a granular Limestone, 
similar to that in which it was found, and which is quarried there for paving, building, &c. 
The perpendicular inner margin of the whorls is three-eighths of an inch on the biggest 
part ; thus the umbilicus is deep, although commencing rather suddenly at the fourth whorl, 
the bottom is consequently nearly flat, composed of four or five whorls, and very perfect, 
to the minutest. I was glad to discover the proper place for the siphuncle, expressed on the 
upper edge, by breaking the shell in looking for the contour of the chambers. 
« This somewhat resembles Amaltheus margaritatus of De Montfort, t. 23, p. 91. The 
place of the siphuncle is of much consequence in distinguishing them, as his is in the middle 
of the inner margin. » 
OBSERVATIONS 
This type is in the Sowerby collection in the British Museum, and is labelled in Sowerby’s 
handwriting « Ammonites excavatum M. C. 105 ». 
The crack approximately marks the commencement of the body-chamber, which, as 
Sowerby observes, is separated from the rest of he shell. 
At a diameter of 8gmm, the ratio of the width of the umbilicus to the diameter is .18, and 
that of the thickness of the last whorl to the diameter is .35, including the test : at a diame- 
ter of 107mm, the ratios are .19 and .33 respectively. 
Genus. — Cardioceras. Neumayr und Uhlig, 1881. 
Horizon. — In the Supplementary Index, p. 249, this species is said to come from the 
Portland Rock, but its true horizon is Lower Corallian. 
Locality. — Shotover Hill, near Oxford. 
1905 M. Healey. 
PALAEONTOLOGIA UNIVERSALIS. — 92? 
