33 



half of the entire diameter. The umbilicus is deeply excavated and 

 concavely conical, especially in tlie centre, but it gets shallower and 

 loses its regularly conical shape near the aperture. As compared with 

 the outer whorl, the inner volutions occupy rather less than one-half the 

 greatest diameter of the shell. As measured from two opposite tubercles, 

 the umbilicus is equal to nearly three-fourths of the whole diameter. 



The aperture is transversely arcuate, and its sides are truncate and 

 subungular. 



The last whorl is ornamented with fourteen, distant, raised, rounded 

 tubercles, which encircle the umbilicus. About as many can be counted 

 on the volution which precedes it, and the coronations can be traced even 

 in the earlier whorls. Where the test is preserved, the periphery and 

 part of the sides are. covered with close-set, numerous and transverse 

 ribs, (varying from a quarter of a line to a line in width) which are too 

 line to leave definite impressions on the cast. These appear to proceed 

 from each of the tubercles in bundles of about ten or fourteen. The 

 distance from the centre of two contiguous tubercles on the outer whorl 

 was found to be about seven lines, and in a sjiace of equal width imme- 

 diately below them, fifteen or sixteen ribs could be counted. These, 

 however, are very unequal in width, even over a very small area, 

 and, of course, are widest near the mouth. 



Clreatest diameter of the shell, four inches and five lines; extreme 

 width of umbilicus, from the centre of two 023posite tubercles, three 

 inches and one line; of the inner whorls, (from suture to suture) two 

 inches and one line. The breadth of the aperture is roughly estimated 

 at two inches and nine lines in its widest part; its height is about 

 eleven lines. 



This interesting shell, of which only one imperfect specimen was 

 collected, is nearly related to the Ammonites coronatus^ of Bruguiere, and 

 A. Blagdenif of Sowerby. The extreme fineness of the ribs in A. Richard- 

 sonii, together with the very slight involution of its outer whorl, will 

 enable it to be distinguished from either at a glance. 



It affords the writer much pleasure to be able to associate the name of 

 its discoverer with this beautiful species. The collection of which it 

 forms a part is only one out of the many additions which Mr. Eichardson 

 has made to our knowledge of the geology and palaeontology of Canada, 

 in a period extending over thirty years. 



» Pal6ontologie Frangaise, Terrains Jurassiques. Vol. I. Atlas. Plates CLXVII I, & CLXlX" 

 t '"Mineral Conchology." Vol. II., page 231. Plate CCI. 



D 



