75 
whorls ; by degrees the ribs that are formed are long 
enough to join the tubercles, and the keel gradually be- 
comes smaller until the ribs are no longer divided by it ; 
lastly, the keel entirely disappears, and the ribs become 
large, broad and blunt. The aperture is about one third 
of the diameter of the shell in length ; sometimes two 
ribs join one tubercle. 
Casts of this variable Ammonite are by no means rare 
in the Sandstone of Blackdown ; the space once occupied 
by the shell itself, which appears to have been thin, is 
tilled by nearly transparent silex, that has, partly in con- 
sequence of its tenuity, a slightly pearly lustre, and 
forms an elegant petrifaction. The difference between 
the young and old shells is so great, that they are gene- 
rally taken for distinct species, but the centre of the old 
shell often shews the form of the young one. 
AMMONITES Turned. 
TAB. CCCCLII. 
Spec. Char. Depressed, radiated, carinated, a 
furrow on ^ch side of the keel ; inner 
whorls exposed ; radii numerous, equal, 
curved towards the front ; aperture oblong, 
quadrangular. 
Volutions about five, the inner ones almost wholly ex- 
posed ; the radii are almost straight until they are rather 
suddenly bent towards the front ; the aperture is less 
than one third of the diameter of the last whorl in length. 
The more exposed whorls, squareish aperture, and 
differently curved radii, distinguish this from A. Brookii, 
tab. 190. to which it bears a strong resemblance. 
The upper figure represents a cast in Pyrites among 
Clay from Wymondham Abbey ; it is probably out of an 
alluvial bed. We are indebted to our kind Friend, the 
celebrated Botanist, Dawson Turner, Esq. for it. The 
lower figure is from a very much compressed specimen, 
in which the shell remains imbedded in a slaty clay. 
Found at Watchet, where this species does not appear 
to be very common. 
