194 GASTEROPODA OF THE INFERIOR OOLITE. 
inconveniently large genus. For instance, in 1850 D. Sharpe divided Nerinzxa 
into four sub-genera, of which Trochalia may be omitted as having no representative 
in our Inferior Oolite. There remain then (1) Nerinella—columella either simple 
or furnished with one fold, the outer lip with one internal fold; (2) Nerinza— 
columella with two or three folds, outer lip with one or two folds, all simple ; (3) 
Ptygmatis—columella usually with three folds, outer lip with one to three folds, one 
or two of the folds of a complex form, either dividing into two lobes or wider towards 
the edge than at the base. D’Orbigny objected to these sub-genera, mainly on the 
grounds that no definite line could be drawn showing where one ended and the 
other began; an objection which, if urged nowadays, would be applicable to most 
zoological divisions. Subsequently both Zittel and Cossmann have more or less 
recognised the value of these distinctions. 
As a matter of fact, in our Inferior Oolite six sections or divisions of Nerinzxa 
may be made out, the distinctions being based upon internal structure. 
Szotton A (Uniplicate). Simple fold on the outer wall; e.g. Nerinella gracilis, 
Lycett. 
Ssotion B (Biplicate). Simple fold on the outer wall and on the columella; 
e.g. Nerinella, species unnamed from Weldon. 
Szotion C (Triplicate). One well-marked fold on the outer wall, one on the 
lower part of the columella, and one about the junction of the columella with the 
posterior wall, all simple, e.g. Nerinxa oolitica, Witchell. ‘This is a very numerous 
section, and includes most of the proper Nerinzas of our Inferior Oolite. 
Section D. Outer wall with sometimes one and sometimes two folds, columella 
with two folds, all simple; e.g. Nerinxa cingenda, Phillips. 
Szotton HE. Five simple folds, in all, on the outer wall and columella; e. g. 
Nerinzxa (? Ptygmatis) Guisei, Witchell. The above species is the only represent- 
ative of this section, which appears to be a sort of connecting link between Nerinea 
and Ptygmatis. 
Srctron F. Outer wall and columella with several more or less complex folds, 
the result being a narrow and whimsical section ; e. g. Ptygmatis Oppelensis, Lycett. 
This is the most numerous section of all. The contrast presented by the internal 
section almost entitles this to the rank of generic distinction. 
General Features of the Nerinxas of the Inferior Oolite—In most cases the 
columella is solid. If there be exceptions, they occur in the upper parts of 
the Lincolnshire Limestone, where a peculiar admixture of forms takes place. 
Where no mention is made to the contrary, it may be taken for granted that 
the columella is regarded as having been solid. Narrow, cylindrical forms 
are the most prevalent. The apical whorls differ greatly from the anterior 
ones (dimorphism). Owing to the indifferent preservation of the surface the 
original character of the ornamentation is somewhat difficult to determine. 
