232 Mr. H. Seeley on Ammonites 



small accessory. The superior lateral lobe is about half the 

 width of the dorsal lobe, and much shorter; the terminal 

 branches are similar, only less developed. 



Only two specimens of this Ammonite have come under my 

 notice. 



The exact affinities of A. Weistii are not quite clear. Speci- 

 mens of A. navicularis come very near to it, but have not the 

 constant greater elevation of occasional ribs ; rather in this it 

 approaches A. peramplus, which, however, has spines in the 

 young state at their umbilical termination. 



Ammonites navicularis, Mant., var. 



The flattened back is slightly inflated, and rounds into the 

 side ; the flattened side, which is also a little swollen, rounds 

 into the umbiHcus. The mouth is higher than wide. The few 

 whorls are almost entirely embracing, forming a deep and small 

 umbilicus about half the diameter of the whorl opposite to the 

 mouth. 



Ornamented with about (thirty to) forty wide rounded ribs, 

 which are straight, strongest where they pass over the back, 

 and separated by spaces of not more than their own width. 

 About half the ribs arise in the umbilicus, the remainder near 

 the middle of the side. 



There are three lateral lobes on each side. The dorsal lobe 

 is wide and square, with three branches on each side, the lower 

 of which have five digits. Dorsal saddle rather wider than the 

 lobe. Superior lateral lobe half as wide as the dorsal saddle, 

 and deeper than the dorsal lobe ; it has two branches on each 

 side and a large terminal one which bifurcates. Lateral saddle 

 like dorsal. 



The few specimens found show considerable variation in the 

 form of the mouth, which is sometimes as wide as high. They 

 are more flattened than is usual in examples of A. navicularis 

 from the Chalk, and difl^r in never having any tubercles ; the 

 umbilicus is also commonly smaller. The Warminster Upper 

 Green sand contains similar shells ; but they have tubercles on 

 the back. 



It may be necessary to separate the Cambridge shell as a 

 variety ; for it is intermediate between A. Weistii and A. navicu- 

 laris, and may be an extremely compressed variety of the former. 

 But A. Weistii can scarcely claim to be more than a well-marked 

 variety of A. navicularis, connecting it with A. peramplus. Our 

 fossils also nearly resemble A. vectensis, Sharpe, and cannot be 

 distinguished as more than a variety, the only differences being 

 that the ribs are straight (seemingly more elevated), the back 

 slightly flatter, and the umbilicus commonly larger. This being 



