GREY CHALK MARL. 115 



I'his species is nearly allied to the preceding, but is distinguished by 

 the ribs in almost every instance, reaching entirely across the volutions ; 

 and by the central row of tubercles on the ambit. 



The sides are flattish, the external margin somewhat angular, the ribs 

 prominent, in most instances entirely embracing the volutions, but in 

 some varieties being alternately short, as in A. Mantelli. They are 

 studded with seven sets of tubercles, arranged in the following manner ; 

 viz. a row on the inner and outer margins, with two intermediate ones on 

 each side ; and a dorsal row along the ambit. The aperture is nearly 

 quadrangular ; and the septa foliaceous. The situation of the siphunculus 

 is unknown. 



In the adult shell the ribs are prominent, and somewhat angular, the 

 dorsum broad and flat, and the central row of tubercles almost obliterated. 



This beautiful species varies from a few inches to a foot in diameter, 

 and was formerly abundant at Hamsey, but is now seldom found. 



Tab. XX. fig. 2, represents a specimen of the size and form in which 

 this species most usually occurs. 



Tab. xxi. fig. 10. A perspective view of a small specimen. 



Localities. Hamsey, Offham, Middleham, Southerham, Rodmill, Plump- 

 ton. 



SO. Ammonites varians. Tab. xxi. fig. 2. 5. 7. 



Discoidal, subumbilicate, volutions depressed, half inserted; trans- 

 versely radiated ; radii bifurcate, undulated, studded with from four to six 

 rows of tubercles ; septa very foUaceous ; carene acute, entire ; aperture 

 sagittate ; siphunculus external ? 



This species of ammonite is one of the most proteiform of the whole 

 genus, presenting great variety in the figure, disposition, and number of 

 the tubercles and costas. It is, however, readily distinguished from its 

 associates, by the acute entire keel, and the bifurcating tubercular radii. 



It is from 0-2 inch to nearly six inches in diameter, and is frequently 

 compressed into an elliptical, and sometimes into a cordiform shape. The 

 volutions seldom exceed four in number, and are rather more than half 

 inserted. The umbilicus is shallow and expanded, its sides smooth, and 



a 2 



