1913 TERMINOLOGY vii 



(Cadoceras) ; a thick stock is becoming thinner (Pachyceras) : there is 

 a median collision point with a maximum of likeness, where the species 

 are homceomorphic and may be (have been) confounded. Stephanoceras 

 sublceve ; Deslongchamps, (Foss. Oxf. ; Notes pal. ; 1889, 1, 1-4) is not 

 Sowerby's species, is not even a species of Cadoceras, but is a Pachyceras. 



Descriptive Terms. (See Vol. 1., p. viii) 



In Mon. Inf. Ool. Amm., 1898, Suppl. p. ii, S. Buckman intro- 

 duced a series of concise terms descriptive of shape and number of 

 whorls, and in p. v terms for umbilication, as follow : — 



f Polygyral with many whorls (slow coiling). 



{ Oligogyral with few whorls (quick coiling). 



J Stenogyral with narrow whorls. 



I Platygyral with broad whorls. 



[ Leptogyral with thin whorls. 



[ Pachygyral with thick whorls. 



Angustumbilicate with narrow umbilicus. 

 Latumbilicate with broad umbilicus. 



In 1903 (Quart. Journ. Geol. Soc. LIX, 461) some of these terms 

 were expanded by the use of per- and sub- into six series of percentages 

 of a base-line, the radius = 100 (See also Mon. I. O. Amm., 1905, p. 

 cxcvii). 



It is obvious that a base-line founded on the radius was correct 

 for the breadth of the whorl > and, in fact, is the only one possible ; 

 because the breadth of the whorl must vary within that limit, reaching 

 possibly to 100 per cent, of such base-line. But a base-line founded 

 on the diameter would give incorrect values for the breadth of whorl, 

 which in an extreme case might reach to no more than some 55 per cent, 

 of such base-line. Then a base-line founded on the radius gave quite 

 satisfactory results for the thickness of the whorl, though, in abnormal 

 cases, the thickness exceeds 100 per cent, of such base-line. But a 

 base-line founded on the diameter, though not giving results so unsatis- 

 factory as in the case of the breadth, would afford less discrimination 

 among 'the numerous thin-whorled forms ; for the longer the base-line 

 the larger the number of whorls which would be called thin. And there 

 is the same objection to a long base-line in regard to the nomenclature 

 of umbilication. All these considerations influenced the choosing of 

 a short base-line such as the radius. 



As, however, a base-line founded on the diameter = 100 has been 

 used in many works for measurements of proportions, it is advisable 

 to retain that base-line for such measurements, in order to compare 

 specimens, without calculation, with those for which proportions have 

 been given. 



