249 



PRESIDENTIAL ADDRESS. 



Delivered 12th February, 1915. 



THE GENUS CLAUSILIA: A STUDY OF ITS GEOGRAPHICAL 

 DISTRIBUTION, WITH A FEW NOTES ON THE HABITS AND 

 GENERAL ECONOMY OF CERTAIN SPECIES AND GROUPS. 



By the Rev. A. H. Cooke, M.A., Sc.D., F.Z.S. 



The genus Clausilia is as interesting a group as any among the land 

 Mollusca. It is "well characterized, and stands, to a certain extent, 

 isolated. To the systematist it offers problems of classification, based, 

 in the main, on an examination of the complicated processes which it 

 has developed for closing the mouth of the shell. To the student of 

 distribution, the sphere and limits of its occurrence, which are well 

 marked, may contribute, if handled with reasonable care, evidence 

 bearing on the question of the ancient connexion of lands now widely 

 separated. 



An authority on the genus, as great as any who have ever lived, 

 Dr. 0. Boettger, regarded Balea, with its sinistral spire, its lack of 

 clausilium, lamellae, and plicae, and its occasional rudiment of a parietal 

 tubercle, as the progenitor of the Clausiliidae, and he considered the 

 living Balea of thepresent day, with theirvery remarkable geographical 

 distribution (Europe, Tristan d'Acunha, South Africa, New Zealand), 

 as salvage from the wreck of the ancient genus strong enough to 

 resist the lapse of ages. Clausilia first appears in the Lower Eocene 

 [Oospira, Fseudonetiia) and Upper Eocene {Disju?ictaria, Albinaria?), 

 and is common in the Miocene {Tripti/chia, Canalicia, Eualopia, 

 Serrulitia, Constricta). Boettger's view was that from an original 

 type possessing neither clausilium, plicae, nor lamellae, the present- 

 day forms, with their elaborate oral armature, developed in more or 

 less regular sequence. In confirmation of this view, he pointed 

 out that there occur, in Tertiary formations, Clausilia without 

 a clausilium (certain recent Alopia being probably relics of these), 

 Clausilia with rudiments of lamellae, or with undeveloped plicae in 

 place of a lunule, and Clausilia possessing other indications of 

 developmental stages, and showing transitions from a less to a more 

 specialized form. 



H. A. Pilsbry, whose views on Clausilia carry great weight, holds 

 (47) that the East Asiatic Clausiliidae {Phcedusa) are much more 

 closely related to early Tertiary than to modern European groups. 

 (Boettger indeed suggests that Eualopia may be the Balea-iorm. 

 originating Phcedusa.) There is reason to believe that, as in the case 

 of the Belogonous Helicidae, a common stock of Clausiliidae spread 

 over Asia and Europe, at least as early as the Eocene period. 

 Subsequent evolution in the East and West has been, he holds, 

 along independent lines, and, as in the Helicidae, the European stock 

 has forged ahead, while the Oriental, on the whole, looks backward, 

 many groups retaining their old characters. 



