St 



shape of the elevations which give a more or less coronated aspect to the 

 shell, varies very little in different examples. Assuming that the whole 

 of these set^en Ammonites belong to one species, it was at first thought 

 difficult to account for the fact that the width (or thickness) of the outer 

 whorl in small individuals was so much greater in proportion to their 

 entire diameter than is the case in more fully grown shells. The expla- 

 nation of this circumstance is very simple. In a specimen at a compara- 

 tively early stage of growth, the whorls are so much compressed on the 

 peripherj' and raised at the sides that the width of the aperture may be 

 twice or even tliree times as great as its height. When more neai-ly ar- 

 rived at maturit}', the whorls become rounded and their sides compressed, 

 the height of the aperture become nearl}^ equal to its breath, and the 

 result is a great addition to the diameter of the shell, which is not accom- 

 panied by a corresponding and proportionate increase of its thickness. 



An appropriate illustration of this peculiar mode of gi-owth is aftbrded 

 by the Ammonites anceps of Reineeke, as figured l)y D'Orbigny in the 

 " Paleontologie Frangaise." In the Atlas to Vol. II, of the ■• Terrains 

 Jurassiques," Plate CLXVI., figures are given of two specimens of 

 A. anceps, one of which is represented as one inch and five lines, and the 

 other as three inches and four lines in diameter. The smallest of these 

 fossils is the thickest of the two, at least if the figures are correct. 



Mr. Billings has suggested * that the large specimen which is here re- 

 garded as the type of A. Skidegatensis, is closely allied to the Perispliinctes 

 ifyrannMSofNeumayr,t from the " Macrocephalen Kalken " of Brielthal. 

 The two species certainly resemble each other in general shajDe and in 

 the amount of involution of the whorls, but their sculpture is sufficiently 

 distinct. The outer whorl of P. tyrannus is said to be ornamented 

 with nineteen distant primary ribs, which trifurcate in passing over the 

 siphonal edge. In A. Skidegatensis the ribs are much more numerous, 

 acute and regular. 



Ammonites anceps of Eeinecke, which also belongs toWaagen's sub-genus 

 Perisphinctes, is still more nearly allied to the present species both in 

 shape and sculptux-e. The only important difference between them is 

 that in the former the ribs are separated by a narrow space which encir- 

 cles the periphery, and this is never the case in A. Skidegatensis. 



* " Geological Survey of Canada. Report of Progress for 1S72-73." Page 72. 



* " .Jahrbuoh der Kaiserlich-Koniglichen Geologischen Reichsaustalt." Vienna: 1870. Vol. XX., page 

 150, Plate IX. 



