L. F. Spatli — A neiv Aminonite from CharmotttJi. 589 



The writer is indebted to Dr. Lang for permission to describe 

 his specimens, also for many valuable suggestions. The writer also 

 hopes to publish in the near future a complete account of the 

 Polymorphidae of the ibex-zone at Charmouth, especially of the 

 successive develoiDments of Acanthojjileuroceras announced by 

 Dr. Lang in his jjajDer. These will ofier a better opportunity for 

 correlating the beds which yielded the new forms described in the 

 present paper. Five of the six specimens mentioned above are 

 considered to belong to one species, but their description is given 

 separately, since, owing to differences in the mode of preservation, 

 crushing, or rolling, each specimen shows interesting peculiarities 

 which supplement the evidence of the holotype. 



The sj)ecimen selected as type is the one that had been exhibited 

 for years in the British Museum (Natural History) as an example of 

 an '' Ammonite replaced by Iron-Pyrites " (specimen No. Z 216). 

 It bears the label " Apparently new. Bought at Lyme Regis, and 

 stated to have been found there. F. H. Butler ". The dimensions 

 are : — 



Diameter ...... 83 mm. 



Height of the last whorl . . .31 °/„. 



Thickness of the last whorl . . .21 %. 



Umbilicus ...... 45 °/o. 



At a diameter of 60 mm. these percentages are respectively : 

 31, 21, 44. The specimen has the (limonitized) shell attached, 

 except on the more prominent parts, such as the crests of the ribs 

 and the peripheral tubercles. This abrasion is probably due to 

 a certain amount of rolling, which would account not only for the 

 rounding of the periphery but also for the apparent duplication of 

 some of the costae. Apart from this, however, there appears to be 

 a flat septum in each costa, which itself was j^robably hollow and 

 very sharp, if one may judge by the other specimens. The ribs 

 may therefore appear flat and broad, after the style of annulate 

 Dactyliocerati, even where there was abrasion of only the crest and 

 tubercle, and where the inner shell or septum still adheres to the rib 

 (Fig. 2). The number of costse is 114 to the whorl at a diameter of 

 83 mm. and about 116 at a diameter of 55 mm. Each begins with 

 a striking backward bend at the umbilical slope (Fig. 4), and describes 

 a sigmoidal curve on the lateral area, ending near the ventral 

 margin with an elongated tubercle or thick ending. In Fig. 3 this is 

 indicated from the evidence of the other specimens ; but in the 

 holotype the periphery is more or less worn throughout. 



The whorl-section is rounded, with its greatest thickness about 

 midway between the periphery and the umbilical suture ; but, owing 

 to the wearing down of the ornament, the sides appear to be flattened 

 on the last whorl ; and, especially near the end, the shell is poorly 

 preserved. From evidence, however, of the inner whorl, it appears 

 that the sides were regularly convex, and this regular rounding is 



