Thompson: The AiJimonites called A. serpentinus. 217 



to the margin, somewhat resembhng the curve of a reaping- 

 hook. The inner edge of the turns is elevated and obtuse. 

 This nearly resembles the last \_A. sirangwaysi], but is not so 

 flat, and wants the fiat surface of the inner margin of the whorl. 

 It is from the Inferior or Iron-shot oolite of Ilminster. 



The following is the description of Harpoceras Strangwaysi 

 (Sowerby sp.), from Mr. Buckman's paper : — ' Discoidal, com- 

 pressed, hollow carinate ; whorls flattened, with genuine sickle- 

 shaped ribs, less conspicuous on body chamber, but there more 

 distinctly bent. Ventral area marked by prolonged sweep of 

 ribs and surmounted by a well-marked hollow carina. Inner 

 margin, almost upright, neither convex nor concave. Um- 

 bilicus shallow, open. Inclusion about one third ; aperture 

 oblong. Sowerby's figure not correct. Inner margin wrong, 

 sectional view wrong. Ribs do not bend enough since they 

 should have a true sickle-shape. His suture lines are right.' 



' Harpoceras Strangwaysi differs from Harpoceras falciferum 

 in having a more open umbilicus, about one-fourth larger ; 

 ribs are not quite so curved, and inner margin is nearly upright 

 instead of undercut. Fish bed, Upper Lias, Byfield, Trent ; 

 Ilminster, Sowerby.' 



A contrast of the characters of the other two types : — 



A. mulgravius. (?) (Of collectors). 

 Plate XIV. 



The shell has a very flat and 

 discoidal appearance. The whorls 

 are broad ; the outer one being in 

 the earlier stages of life nearly half 

 the diameter of the shell. In early 

 life, too, each whorl is rather deeply 

 indented by the preceding one, but 

 one of the characteristics of the 

 species is, that the amount of inner 

 whorl covered by the outer one con- 

 tinually decreases with age, until 

 in the last stage the body whorl 

 covers a little more than a quarter 

 of the preceding one. 



Therefore the relative size of 

 the umbilicus to the whole shell is 

 continually increasing with age. 



In consequence of the flatness 

 of the inner third of each whorl, the 

 umbilicus descends to the centre 

 by a number of broad flat steps. 



The inner margin of the whorl 

 is also characteristic, being under- 

 cut, or, if it be preferred, it slightly 

 overhangs the preceding one. In 



Hildocerns serpentinum.* 

 Plate XIII. 



This also has a flattened and 

 discoidal appearance, but the whorls 

 are not so broad as in mulgravius, 

 nor Sowerby's jalcifer ; roughly 

 three-quarters for the same sized 

 shell. 



The umbilicus is large and open, 

 for the whorls enclose very little 

 of each preceding one. The coiling 

 is regular ; that is, the amount of 

 involution does not vary with age 

 as in the species compared with it. 



The inner margin is obliquely 

 flattened, the slant being down 

 towards the centre of thfe shell, not 

 awav from it. 



Description mainly from Mr. Buckman's paper. 



igog June i. 



