24 FOSSIL MOLLUSCA OF THE CHALK. 



A. testa discoidedy aliquando sub-infatd, costatd, tuber culatd, carinatd; costis inter- 

 ruptisy Bimplicihus vel furcatis, tri-vel quatuor-tuberculatis ; tuberculis prominentibus ; dorso 

 latiuscido, utrinque tuberculato, medio carinato ; wmbilico profunda ; aperturd truncato- 

 ovatd. 



Shell discoidal, with the whorls inflated when young, but somewhat flattened when 

 adult, ribbed, and tuberculated ; back broad and keeled, with a row of prominent tubercles 

 on each side ; aperture ovate, with the upper part of the whorl projecting forward, and 

 produced into a blunt point ; umbilicus deep and rounded off" at its upper edge. The ribs 

 and tubercles vary much both in size and number at different stages of growth and in 

 different varieties of the species. Septa with four trifid, sub-divided lateral lobes. 



This species admits of a division into two strongly-marked varieties, which Haan 

 regarded as distinct species : — 



1st Var. iubercidata of Man tell, whorls of the young shell (PI. VHI, fig. 4,) inflated, 

 their width being equal to their height, with three rows of very prominent tubercles con- 

 nected by thick, ill-defined ribs ; the tubercles of the inner row round the edge of the 

 umbilicus are the smallest, ten in number in a very young shell, twelve in a shell of 1^ inch 

 in diameter ; the tubercles of the next row, situated on the middle of the whorl, are very 

 large and prominent, and equal in number to the preceding ; the tubercles of the outer 

 row on the edge of the back are of an intermediate size, and about half as many again as 

 the others; in the adult sheU (PL VHI, fig. 2; PI. IX, fig. 1,) the whorls are a little 

 flattened, the ribs better marked, the tubercles rather less prominent, and a fourth row of 

 small tubercles is inserted between the middle and the dorsal rows; the width of the 

 aperture is equal to yV^^s of the height of the whorl. The usual diameter of this variety is 

 about 2^ inches, but specimens are occasionally found in the Grey Chalk of 6 inches 

 diameter : this enlargement is not due to an increased number of whorls, but to a larger 

 size at every stage of growth. 



2d Var. injlata, PI. VIII, fig. 1, whorls inflated, their width being one fourth more 

 than their height, with two rows of very large and prominent tubercles ; those of the 

 inner row which are seated on the middle of the whorl being the largest, and ten or 

 eleven in number ; the outer row on the edge of the back somewhat smaller, and sixteen 

 or seventeen in number ; back broad, with a very prominent keel. This variety reached 

 2\ inches in diameter, and 1^ of an inch in thickness. 



Ammonites Coupei is common in the Chloritic Marl of the Isle of Wight, and in the 

 Chalk with siliceous grains of Dorsetshire ; and is found sparingly in the Grey Chalk of 

 the Southern Counties. 



The original description of A. varians in the ' Mineral Conchology' included our 

 A. Coupei, which was first separated by Brongniart : most subsequent authors have re- 

 garded them as distinct species, until they were reunited by M. D'Orbigny, who con- 

 siders that all the varieties of the two species pass into one another, and may in part be 

 accounted differences of sex, the more inflated forms being regarded as females. We 



