392 GASTEROPODA OF THE INFERIOR OOLITE. 
The various zones of the Inferior Oolite in Yorkshire are almost equally destitute 
of Plewrotomariex, a very few specimens of a large form, related to the Liassic 
species, Pl. Anglica, being found in the lower portion of the Dogger. 
Our collections, therefore, have received the bulk of their supplies from the 
Inferior Oolite of Dorset-Somerset, including Dundry, Most of the species 
recorded from these beds bear more or less resemblance to those already so well 
described and figured by Deslongchamps and d’Orbigny. Consequently it is 
proposed, in some cases, to deal rather briefly with the descriptions in the text. 
One of the points of interest which I have been able to elucidate relates to the 
distribution of the particular species in the Dorset beds. It is also clear that the 
several ‘ species” have a tendency to run into each other, and thus gradually to 
merge or to become something requiring a different diagnosis. In no other group 
of the Gasteropoda has the lesson been enforced upon me more strongly that the 
mere enumeration of named forms, which we regard as species, is inadequate to 
convey a complete idea of the actual facts. 
As regards the sections or groups into which the Inferior Oolite Plewrotomarie 
most naturally fall, opinions may to a certain extent differ. Again reverting to 
the two principal authors who have dealt with the subject, we perceive in the 
pages of Deslongchamps a systematic arrangement applied to the entire Jurassic 
Pleurotomarize of Normandy, whilst d’Orbigny appears to have made very little 
attempt at classification in any respect. Deslongchamps’ treatment of the subject 
was much the most philosophic, and although he lived before the days when the 
doctrine of Evolution had been revealed to mankind, it is evident that he had some 
inkling of it. On the other hand, if ever there was a man who thoroughly 
believed in species, especially of his own creation or rectification, that man was 
the author of the ‘Terrains Jurassiques.’ His method undoubtedly has its 
merits, and the rigidity of his species is convenient for the collector. 
In submitting the following grouping, which does not differ very widely from 
that adopted by Deslongchamps, I must again express my belief that sections, 
groups, and species must be regarded merely as means to an end, viz. the 
presentation of certain paleontological facts in what seems to be the most natural 
form. That there is a certain amount of inconvenience in appending a long string 
of varieties to certain ‘ species” I admit, and some would, perhaps, prefer the 
more rigid d’Orbignian method; but if this is adopted the number of species 
would be greatly increased. 
