392 GASTEROPODA OF THE INFERIOR OOLITE. 



The various zones of the Inferior Oolite in Yorkshire are almost equally destitute 

 of PleurotomarisB, 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 Pleurotomarise 

 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 

 Pleurotomarise 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 palasontological 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. 



