11* GREY CHALK MARL. 



cles are placed on every rib, and form a row on each margin of the ambit, 

 or back, of the shelL 



Var. tuberculo-costata : with six rows of tubercles. This variety, in 

 addition to the marginal tubercles, has four rows, which are placed on the 

 longer costae only ; each side of the shell having one set on the margin of 

 the umbiHcus, and another at a short distance above it. 



Var. tuberculata : with eight rows of tubercles. The two additional 

 sets which distinguish this variety, are placed one on each side, midway 

 between the margin of the ambit, and the second row of tubercles from 

 the umbiUcus. These intermediate tubercles occur on every rib, each of 

 the larger costae being ornamented with eight, while the shorter ones have 

 but four. From the numerous tubercular projections on this variety, the 

 outer volution is somewhat pentagonal. 



The septa of A. Mantelli are numerous, and very foliaceous. The 

 form of the aperture varies in different specimens, but its width is in 

 general equal to about two-fifths of the diameter of the shell. The 

 siphunculus is small, and extends along the centre of the ambit. 



This species frequently attains a large size, some specimens exceeding 

 one foot and a half in diameter ; but in these the tubercles are almost 

 obliterated. 



Tab. xxi. fig. 9. A beautiful cast, from Middleham, of the first variety. 



Tab. xxii. fig. 1 . This specimen also belongs to Var. costata * ; it exhibits 

 the foliaceous septa, and the situation of the siphunculus. It was col- 

 lected by my friend, Thomas WooUgar, Esq. of Lewes. 



LocaHties. In almost every spot in Sussex, where an excavation has 

 been made in the grey marl. 



29. Ammonites Susseociensis. Tab. xx. fig. 2. tab. xxi. fig. 10. 



Discoidal, subumbihcate : volutions subquadrangular, inserted, costated; 

 costae transverse, numerous, with seven rows of tubercles, one of which 

 extends along the centre of the ambit ; septa foliaceous ; siphun- 

 culus ? 



* The plates of this work were completed before any illustrative specimens of the other 

 varieties had been discovered. 



