226 Mr. II. Sccley on Ammonites 



hamus, which may indicate that, after these had become filled 

 with phosphate of lime, the partitioned part was broken off and 

 floated away at the surface, much as the recent Spirilla is found 

 drifting. Perfect shells are rare, and young ones never found. 



There are three variations from this form : one, which is 

 ornamented with a row of tubercles on each side, is also found 

 in the Gosau Chalk. 



Ammonites rostratus, Sow. 

 Ammonites rostratus, Sow. A. symmetricus, Sow. 



Numerous citations have been given by Continental writers, 

 to whom the species seems scarcely known, being regarded as 

 synonymous with A. inflatus, Sow. 



Shell keeled, few-whorlcd, with a flat back, flat sides, quadrate 

 mouth, and umbilicus becoming relatively smaller with age, but 

 never less than the height of the body-whorl ; not very deep, it 

 is angular, though not making a sharp angle with the side. 

 The back is generally a little narrower than the side is high ; 

 out of its middle arises the narrow keel, from each side of which 

 the ribs are directed backward and outward at an angle of 45°, 

 then descend straight, elevated, perpendicular, and separated by 

 wide spaces, and terminate, generally separate, in tubercles at 

 the base. On a whorl there are from 25 to 35 ribs, each 

 having a tubercle above the middle, and another where they 

 reach the back. The wide intercostal spaces are smooth. When 

 full-grown, the last two or three ribs incline frontwards; the 

 keel and adjacent lateral parts of the back become elevated, and 

 are directed upward and forward in a curve to form the rostrum, 

 which is hollow and rhomboid in section ; for the last rib but 

 one, instead of dying away on nearing the keel, as those behind 

 it do, is continued up its side, becoming less and less distinct. 

 The whole whorl becomes narrow. This rostral prolongation is 

 open in front, and in its upper third curved slightly down. 



The septa are symmetrical. The dorsal lobe is longer than 

 wide, extending between two tubercles, and being margined by 

 the I'ising of the ribs. It has three notches on each side, the 

 two terminal of which become with age digitated branches. 

 The dorsal saddle is relatively very wide, extending down to the 

 lateral tubercle ; it is doubly cleft, the upper accessory being 

 much the larger, above the middle, and in a line with the dorsal 

 tubercles. The upper lobe gets relatively wider with age, and is 

 from a third to more than half the width of the dorsal saddle ; 

 it has a branch on each side, and two terminal branches digi- 

 tated with age. The lateral saddle is slightly wider than the 

 superior lateral lobe. The lower lateral lobe is on the ventral 

 angle, and very small. 



