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 

 filled 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 Turneri. 



TAB. CCCCLIl. 



Spec. Char. Depressed, radiated, carinated, a 

 furrow on each 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 appeaf 

 to be very common. 



