from the Cambridge Greensand. 227 



Besides this, there are four rare forms varying from the type 

 chiefly in the degree of development or suppression of the ribs 

 and tubercles. 



This Ammonite is found on every digging along a line of 

 thirty miles, and is the most abundant. The whorls are always 

 coiled on the dorsal tubercles or the homologous thickenings of 

 the ribs ; so that the size of the umbilicus depends on the rate 

 of increase in the height of a whorl. As a general rule, the 

 more compressed the shell, the closer the ribs. 



In a very young state, the whorls are smooth and round, and 

 continue so for three or four whorls, and then rapidly assume 

 the typical form ; and as this part appears devoid of septa, it 

 might have been formed in the egg. 



Large specimens are very rare, and the largest found are 

 much inferior in size to those of the Gault at Folkestone or the 

 Upper Greensand of the Isle of Wight and Devizes, though 

 there can be no doubt that some Cambridge specimens reached 

 a diameter quite as great as that of their southern brethren. 

 Part of a whorl with the rostrum attached is only 1^ inch high. 

 Broken rostra are comparatively common ; and it can only be 

 supposed that the large shells to which they belonged, having 

 but weak septa and being filled with phosphate of lime, were 

 broken up before fossilization. The result was not the production 

 of the numerous small examples ; for, as the last half whorl is 

 almost invariably devoid of partitions, these died small if not 

 young, and there is no evidence that a rostrum was ever formed. 



It is probably the true A. symmetricus of Sowerby; but that 

 type is seemingly now lost. 



Ammonites pachys. PL XI. fig. 4. 

 Keeled, few-whorled ; whorls flattened on the sides and back, 

 so as to appear in section nearly quadrate [about half-embracing] , 

 Umbilicus as high as the back is wide, and higher than the side 

 of the last-formed chamber ; it is deep, with the ventrum nearly 

 at right angles with the side of the whorl, where it is rounded : 

 this space is marked (at a diameter of 2\ inches) with about 

 twenty-two strong rounded ribs descending from as many large 

 spinous tubercles which margin the base of the side. From 

 each tubercle two ribs (rarely three) ascend the side of the 

 whorl, at first but little elevated, but higher and wider near the 

 back. The interspaces are never wider than the ribs, and on 

 the back are narrower ; on the sides, owing to a curve in alter- 

 nate ribs, they are equal. On the dorsal surface the ribs are 

 curved anteriorly, and become obliterated near the keel. On the 

 back (and in old specimens on the sides too) the ribs are crossed 

 transversely by small elevated lines placed rather close together. 



16* 



