THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[THIRD SERIES.] 

 No. 94. OCTOBER 1865. 



XXVII. — On Ammonites from the Cambridge Greensand, 



By Harry Seeley, F.G.S., of the Woodwardian Museum, 



Cambridge. 



[Plates X. & XL] 



Ammonites [Scaphites) aqualis, Sow. 



Shell much inflated, convex, with a wide back, and the convo- 

 lute portion so coiled as not to produce an umbilicus : this part 

 forms about half the length of the shell, and is always half the 

 width of the back, or wider. The back is about twice as wide 

 as the side, and less convex. When the whorl recurves and 

 forms the mouth, it contracts. 



Both spire and hamus are marked with fine elevated ribs, 

 which are most elevated on the sides, and bifurcate before cross- 

 ing the back. On the spire they are curved slightly away from 

 the mouth, so that the lines are concave in front ; on the hamus 

 they pass over straight, and are separated by wider concave 

 channels. 



The symmetrical septa consist of a rather small square dorsal 

 lobe, with two small notches on each side, and two digitated 

 terminal branches. The dorsal saddle is enormously wide, ex- 

 tending to the limit of the back, where the ribs bifurcate ; it is 

 centrally cleft by a branch half as large as the dorsal lobe. The 

 superior lateral lobe is as wide as half of the dorsal saddle, has 

 one small notch on each side, and terminates in two large 

 branches, which bifurcate, are digitated, and are near together. 

 The inferior lateral lobe is small, and at the base of the side. 

 On the ventrum are the ventral lobe and four pairs of acces- 

 sories. 



The Scaphita of the Cambridge Greensand are abundant in 

 individuals, though few in forms. Authors have generally, and 

 perhaps rightly, referred similar fossils to the S. aqualis of 

 Sowerby. 



Nearly all the specimens found are the last chamber, or 

 Arm. Ss Mag. N. Hist. Ser. 3. Vol, xvi. 16 



