OOLITIC HOCKS. 
75 
In addition to the measurements of individual specimens, I 
have, on the suggestion of Colonel Strachey, appended to the 
description of each species, the relative dimensions of the parts 
of the shell. In so doing, I do not wish it to be inferred 
that the ratios given are invariable for each species; indeed, 
I am convinced that as regards the relative thickness, diame¬ 
ter of the whorls, &c., considerable variation exists in most 
cases ; but comparative measurements, such as those given, 
afford more useful data for the determination of species, than 
the mere absolute dimensions of the specimens described. 
The ratios selected are :— 
1st.—That of the radius vector of the ultimate and penul¬ 
timate whorls, or of the two radii measured from the centre 
of the spiral to the external edge of the aperture, and to the 
termination of the second whorl respectively. 
2nd.—That of the portion of an inner whorl concealed by 
the overlap of an outer whorl, to the diameter of the latter. 
3rd.—That of the thickness of a whorl to its diameter, the 
latter being in all cases understood to be the measurement 
taken on the plane of the spiral. 
P. S.—[in reprint]. Since these pages were written. Professor Dr. Oppel, 
of Munich has favored me with a copy of his description of the Ammonites 
collected at Spitih by the late Herr Adolp von Schlagintweit. Some of these 
species are identical with those here described, and my own nomenclature must 
therefore yield to that of Dr. Oppel, whose work enjoys priority of publication. 
Others of Dr. Oppel’s species appear to be identical with those of Dr. Gerard’s 
collection, described by myself in the Journ. As. Soc. Bengal, 1863, p. 124. 
All these synonyms will be found enumerated in a note at the end of this re¬ 
print. 
Some corrected references to my paper in the Journ. As. Soc. have been 
inserted in this reprint. 
