242 Mr. H. Seeley on Ammonites 



high. The sides are very slightly inflated, and converge so that 

 the back is only half as wide as the lower part of the mouth ; 

 they round into the umbilicus, bordering which are ten slightly 

 elevated tubercles, forming the boundary for the embracing 

 whorl. The flat back has on each dorsal angle a row of eighteen 

 or nineteen tubercles, which alternate, are larger than those of 

 the umbilicus, and are directed slightly upward, though scarcely 

 rising above the back. From the tubercles arise flexuous ribs 

 twice or three times as numerous as the dorsal tubercles, at which 

 they meet commonly in twos, sometimes in threes, with usually 

 a free rib between each two tubercles ; they are similarly col- 

 lected at the umbilicus. The degree of elevation of the ribs, 

 which are sometimes indistinct, varies much, as does the degree 

 of flexure. 



The septa are unsymmetrical, with a square dorsal lobe 

 having two branches on each side, and two terminal branches, 

 between which are a row of dorsal tubercles. The dorsal saddle, 

 wider than the lobe, is cleft mesially by a branch in a line with 

 the other row of dorsal tubercles. The superior lateral lohe is 

 longer than the dorsal, has on each side two lateral branches, 

 and terminates in three branches larger than the others. There 

 do not appear to be any accessory lobes. 



From this, forms diverge having the septa variously sym- 

 metrical and unsymmetrical, in which the whorl is thicker, while 

 the umbilical tubercles are elevated into spines, and those of the 

 back become higher and wider. These are more typical forms of 

 A. auritus, about intermediate between the figures of Sowerby 

 and D'Orbigny. 



Ammonites Raulinianus, var. 



This is quite inseparable as a species from A. auritus. 



Shell inflated, with a flat back tuberculate at the sides; 

 whorls half-embracing. Umbilicus nearly as high as the mouth, 

 and bordered with spines. 



Mouth as wide as high, with the sides slightly converging to 

 the back j in their lower third they round into the umbilicus, 

 and on its margin support a row of nine or ten spines, generally 

 large, but varying. The back has about eighteen tubercular 

 spines, larger than those of the umbilicus, sometimes directed 

 upward, sometimes outward. The rows on the two sides are 

 alternate, so that the ornament of the back is zigzag. The ribs 

 are commonly strong and obtuse, and slightly curved forward. 

 Three always diverge from each umbilical spine, and two always 

 meet in each dorsal tubercle. Occasionally a dorsal tubercle 

 sends down a free rib. 

 . Septa nearly symmetrical, with a square dorsal lobe, narrower 



