262 



THE CLAUSIUM IN ALOPIA, A SUB-GENUS 

 OF CLAUSILIA. 



By J. DAVY DEAN, 

 Of the Department of Zoology, National Museum of Wales 



(Read before the Society, March 14th, 1917). 



Plates 5 and 6. 



Introduction. 

 The study of the clausium in Clausilia appears to have received but 

 scant attention from English writers. The best account I have seen 

 is the one in Dr. Gray's edition of Turton's Manual/ i" which the 

 British species are classified according to the character of the 

 clausium. Lovell Reeve," writing a few years later, describes the 

 clausium as a calcareous appendage or spoon-shaped lamina con- 

 forming to the contour of the aperture, but he does not enter into 

 specific differences. Of modern writers, perhaps Mr. John W. Taylor 

 deals most prominently with the clausium of the British species, 

 but I have not been able to find anywhere detailed comparisons 

 with continental types or forms without which a thorough under- 

 standing of our own species seems impossible. In this connection 

 a study of the clausium in Alopia has a relative importance. Indeed, 

 it is almost indispensable. 



Dr. Gray's classification of the British species of Clausilia is as 

 follows : — 



a. Clausium notched at the tip, fitting into a plait on the outer 

 lip of the shell ; shell smooth {Alarpessa Gray). 



1. Clausilia bidens (Miiller) (= laminata Montagu). 



b. Clausium entire at the top \ shell corrugated (^Iphigenia Gray). 



2. Clausilia biplicata (Montagu). 



3. Clausilia rolphii Gray. 



4. Clausilia nigricans (Maton and Rackett) 



(= bidentata Strom). 

 Kobelt in his Catalogue of European Land and Freshwater Mol- 

 lusca,^ divides Clausilia into twenty-seven sections, including Alopia ; 

 whilst Dr. Westerlund in the " Monographia Clausiliarum " ■* gives 



1 Manual of the Land and Freshwater Shells of the British Islands, by William Turton ; 

 revised edition by Dr. Gray, 1S57. 



2 Land and Freshwater Mollusks, by Lovell Reeve, 1S63. 



3 Catalog der im europaischen Faunengebiet lebenden Binnenconchylien von Dr. Wilh. 

 Kobelt, 1881. 



4 Monographia Clausiliarum in regione Palscoarctica Vivenlium, scripait Dr. Carl Agardh 

 Westerlund, 1878. 



