from the Cambridge Greensand. 239 



Septa commonly unsymmetrical, though the degree of in- 

 equality varies. The dorsal lobe is square, with two digitated 

 terminal branches, and two or three small notches on each side. 

 The dorsal saddle is half as wide again as the lobe, and mesially 

 cleft, though not deep. The inferior lateral lobe is almost as 

 large as the dorsal saddle : it terminates in three large branches, 

 all well digitated, the lateral ones bifurcating in full-grown 

 forms, but not into equal parts. There are three other lobes, 

 which are mere notches. The septa are very close together. 



I have referred this fossil to the A. splendens of Sowerby 

 rather than to the A. Fittoni of D'Archiac, because it is quite 

 identical with typical specimens from Folkestone, though, were 

 Sowerby' § figure followed, no doubt it should be named A. Fit- 

 toni. But for the mineralization, it might have been supposed 

 that ours were southern Gault fossils, the only difference being 

 that, from the smaller size of the crenulse, the back is commonly 

 a little convex instead of being slightly concave. Nothing ap- 

 pears to be gained by separating A. Fittoni from A. splendens-, 

 for it is not a well-marked variety, and our specimens are 

 slightly intermediate. It is a common fossil, and abundantly 

 represented in all collections, particularly those of the University 

 and Mr. Carter. 



One variety, for which I am indebted to Mr. C. S. P. Darroch, 

 of Trinity College, has the septa at first slightly unsymmetrical, 

 and afterwards symmetrical. The shell is inflated, the mouth 

 being two-thirds as wide as high. The rather deep umbilicus 

 is bordered at the ventral angle with sixteen round tubercles. 

 The dorsal tubercles are larger than those at the umbilicus. 

 The back is round. The sutures are the same as in A. splendens, 

 except that there are three small lobes in the umbilicus instead 

 of two, while the lateral lobes are relatively only half as wide. 



There are many variations of A. splendens, through which the 

 smooth forms pass into others having a sharp and elevated 

 flexuous rib descending from each small dorsal crenulation to 

 the base of the side, where two commonly unite to form a slight 

 thickening ; between each two is a free rib, commonly not 

 descending so far. Occasionally two ribs unite in one dorsal 

 tubercle. 



Some specimens reach as large a size as those from the Gault, 

 and must have had a diameter of 7 or 8 inches, but are only 

 found in fragments. 



Passing on from these forms, the ribs gradually get less 

 sharp and wider apart, the umbilical thickenings more elon- 

 gated, and unite three ribs with intermediate free ones. Two 

 unite more commonly in each dorsal tubercle, which becomes a 

 trifle larger. The whole shell gradually thickens, the umbilicus 



