200 UPPER, OR FLINTY CHALK. 



concealed ; the external whorl equal to four-sevenths of the diameter of 

 the shell ; carene very narrow, rounded ; aperture acutely sagittate. 



This ammonite may be readily distinguished in a suite of specimens, 

 although its characters are rather of a negative description. In its 

 general form it resembles A. complanatus, {Grey Marl Fossils, No, 34;) 

 but the umbihcus is larger, the carene less acute, and the surface exhibits 

 no traces of striee, or pHcee. 



Its longest diameter is commonly about fourteen inches ; width of the 

 outer volution nine inches ; greatest thickness five inches ; at the external 

 edge, one inch and a half. The volutions are but few, and with the ex- 

 ception of four or five obscure, transverse, radiating ridges, are perfectly 

 even. The greatest thickness is at the inner margin, from whence the 

 wreaths gradually taper to the keel. The outer volution increases rapidly, 

 and is nearly equal to half the diameter of the sheU. 



In the specimen figured, which is nearly one foot and a half in 

 diameter, the volutions appear to be wholly inserted, but probably in 

 more perfect examples, their inner margin is exposed. The septa are 

 sinuous, and very numerous ; and the surface of the specimens is generally 

 covered with thin, foHaceous impressions, which closely resemble those of 

 the "ammonite articulee" figured by Fauj us St. Fond, (PI. xxxi.FIist. 

 Nat. St. Pierre.) 



Locality. Lower chalk, near Lewes. 



60. Ammonites peramplus. 



Discoidal, subumbihcate ; volutions subcylindrical, nearly half inserted ; 

 a row of indistinct, distant, oblong eminences, on the inner margin ; aper- 

 ture transversely obovate. 



This is a very large species, frequently exceeding two feet in diameter; 

 it is seldom found entire. The longest diameter of a specimen in my 

 possession is 24 inches ; shortest diameter 1 8 inches ; circumference of 

 the outer volution, at the aperture, 23 inches ; diameter of the umbilicus 

 7 inches ; depth of the same 3*5 inches. 



The volutions are four or five, almost half concealed, and in all 



