2i8 Thompson : The Ammonites called A. serpentinus. 



the very young, the whorls are 

 elliptical in section, then for a very 

 short distance the inner edge is 

 vertical, but soon it becomes over- 

 hanging, and the section can no 

 longer be said to be elliptical. 



A little less than half-way across 

 each whorl is a well-marked longi- 

 tudinal furrow. On the ventral 

 area (popularly ' the back ') is a 

 well-defined septate keel without 

 bordering furrows. A septate keel 

 is one which is separated from the 

 chamber by a ribbon of shell, so 

 that when the keel is knocked off 

 the ventral area appears rounded. 

 This character is well shown by 

 many specimens. 



The ornamentation is also 

 characteristic. The ribs are dis- 

 tinctly sickle-shaped. In the young 

 there is a stage in which there are 

 short primary ribs which bifurcate 

 but soon they become single, broad 

 and flat. Passing from the um- 

 bilical edge, they curve gently 

 forward, then at the groove, take 

 a sudden bend backward, and at 

 once sweep round in a prolonged 

 curve towards the mouth of the 

 shell, becoming more prominent, 

 broader and rounder as they do so. 

 They then die out at the base of the 

 keel. 



In the adult form when the shell 

 is preserved, the ribs are seen to 

 begin at the edge of the umbilicus, 

 almost as harrow elevated lines, 

 each alternate one developing the 

 above character, while the others 

 proceed only about half or two- 

 thirds across the whorl as quite 

 subsidiary, or intervening ribs. 

 This character is not visible in the 

 cast. 



In well-preserved and carefully 

 cleaned specimens, the ribs are seen 

 to be present on the slanting wall 

 of the umbilicus, and pass backward 

 at an angle of about 30°, but turn 

 sharply forward on the very edge 

 of the whorl, to take the direction 

 given above. 



The suture lines are very much 

 foliated, being deeply indented by 

 narrow accessory lobes. They 

 crowd one on the other, so that even 

 in young shells no larger than 30 

 mm. they actually overlap. 



The external or ventral saddle 

 has the prominent deep accessory 

 lobe characteristic of the genus 

 Harpoceras. 



Has no longitudinal furrow on 

 the sides of its whorls. 



The keel is non-septate, so that 

 the mould is exactly the same 

 shape as the shell ; the mud filling 

 the keel which is entirely open to 

 the chambers. Two slight furrows 

 border the keel, but die away on 

 the body whorl. 



The ribs are sigmoidal, almost 

 exactly like the letter ' S.' Not 

 very plain on inner margin. 



The suture . lines are much 

 simpler, they are rather remote, 

 or distant from one another, and 

 resemble those of ' hifrons ' very 

 closely. 



Naturalist, 



