from the Upper Greensand of Hunstanton. 175 
The back is hollow—not ploughed, to use a carpenter’s term, 
as in A. faleatus, but with a central concave channel, shallow and 
wide, occupying a third of the width. In some varieties it is 
margined with distant slightly elevated tubercles. 
Specimens in which the mouth is wide have the ribs more 
strongly marked, and continued to a larger diameter. About five 
appear to collect in each tubercle, and one or two between, all 
dying off insensibly towards both back and umbilicus. In these 
thicker forms the sides converge more rapidly near the back. It 
is essentially an inflated form of Ammonites lautus in which the 
ribs and tubercles are obliterated. 
In the more inflated varieties the umbilicus becomes very large, 
and the sides round into it. In these the base of the side is mar- 
gined by eight or nine rather sharp large tubercles. In a young 
state there were on the sides many close straight nbs, but they 
disappear with a diameter of 2 inches. 
The septa are complicated. The upper lateral lobe is much 
notched and digitated, and divided by a five-fingered branch into 
two parts, of which the outer one is the longer. The lateral 
saddle, in the middle of the side, is large, has one major branch 
on each side much digitated, and behind it two or three minor 
branches, which probably vary with age: it terminates in a large 
central branch, with three or four fingers on each side and one 
in the middle, all well notched. There are two inferior lateral 
lobes and an inferior saddle. 
Diameter 6 in.; greatest height of mouth 22in., height of side 
24 in., height of umbilicus 2? in., height of whorl opposite 
mouth 27in. The back has no real limit, but may be stated as 
an inch wide. 
I am not acquainted with any Cretaceous Ammonite having a 
channeled back and smooth sides. Hence this is readily distin- 
guished from all described species. A. solenonotus, mihi, of my 
list in the ‘ Annals,’ Oct. 1864, was the name then used for the 
inflated, ribbed, tubercled variety of this species. 
Ammonites spherotus (Seeley). 
A compressed shell with flat converging sides, a round back, 
and small umbilicus; devoid of ornament. Height 4 inches, 
height of mouth 24 inches, greatest height of umbilicus 2 inch ; 
width of base of mouth 154, inch, width of back 2 inch. 
The small umbilicus is bordered by a flat and but slightly in- 
clined periphery, which makes a sharp angle with the side. Tbe 
inner halves of the sides are nearly parallel, and converge 
slightly, but the outer halves converge more rapidly. The width 
of the base of the mouth, where the shell is half a whorl smaller, 
is 11 inch, and the back is relatively wider than at the larger 
