194 GASTEROPODA OF THE INFERIOR OOLITE. 



inconveniently large genus. For instance, in 1850 D, Sharpe divided Nerinsea 

 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) Nerinsea — 

 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 Nerinsea 

 may be made out, the distinctions being based upon internal structure. 



Section A (U7iiplicatas). Simple fold on the outer wall; e. g. Nerinella gracilis, 

 Lycett. 



Section B (Biplicatse) . Simple fold on the outer wall and on the columella; 

 e. g. Nerinella, species unnamed from Weldon. 



Section C [Triplicatx) . 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. Nerinsea oolitica, Witchell. This is a very numerous 

 section, and includes most of the proper Nerinseas of our Inferior Oolite. 



Section D. Outer wall with sometimes one and sometimes two folds, columella 

 with two folds, all simple; e.g. Nerinsea cingenda, Phillips. 



Section E. Five simple folds, in all, on the outer wall and columella; e.g. 

 Nerinsea (? Ptygmatis) Guisei, Witchell. The above species is the only represent- 

 ative of this section, which appears to be a sort of connecting link between Nerinsea 

 and Ptygmatis. 



Section 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 Nerinseas 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 diflficult to determine. 



