145 



AMMONITES Plicomphalus. 

 TAB. (XCCIV. 



J. HIS figure repiesents the youni^ state, or what 

 amounts to the same thiiii>-, the inner whorls of an Am- 

 jnonite formerly given at tab. 359, but has so different 

 an appearance, that it would not be suspected to be the 

 same, were they not found united in one specimen, which 

 has been proved by breaking- the one first figured ; and 

 shews the necessity of adding to the specific character 

 that the front is, in the young state transversely fur- 

 rowed. This specimen is also irom Bolingbroke. 



AMMONITES miitabilis. 



TAB. CCCCV. 



Spec. Char. Depressed; outer whorls com- 

 pressed, plain and smooth ; inner whorls 

 two thirds exposed, tuberculated, plicated; 

 plicee interrupted over the front ; aperture 

 ovato-sagittate. 



J.N the young state, the front of this xVmmonitc, has a 

 narrow, flattened space around it, from which nunierous 

 very neat plicge spread half way over the sides ; next to 

 these, a single row of compressed tubercles is placed, 

 the inner margin of the whorl is smooth : when the shell 

 is about two inches in diameter, the tubercles cease to be 

 formed ; but the plicae continue until it is six inches 

 over, Avhen they also gradually disappear, the front be- 

 comes rounder, and the sides quite plain, and com- 

 pressed rather obliquely towards the front, so as to give 

 the aperture a blunted sagittate form ; the septa are 

 rather close, and sharply sinuated ; the shell pearly; 

 there are at distant intervals contractions in the whorls, 

 that have been probably formed by an inflected or thick- 

 ened lip, at successive periods of growth. 



The figure? repre-sent a small individual shewing the 

 ornamental part of the shel], and a portion of a large 

 specimen, measuring ten inches in diameter, and of 

 which the greatest thickness is an inch and an half; I 

 am indebted to George Wier, Esq. for them, they were 

 produced by the Clnnch Clay near HorncastJe. 



There appears to be no regular rule amongst Am- 

 monites for their chiuige o( form, some becomin*^ more 



