DEAN : THE CLAUSIUM IN ALOPIA. 263 



twenty-three sections. I have examined species in the greater number 

 of these sections, and am convinced that the presence of the notch 

 indicates an important phase in the development of the 61ausium. 

 The '• entire " clausium is the prevaiHng type, and its purpose is to 

 close the aperture of the shell completely. The notched type may 

 correspond in function to the imperfect epiphragm noticed in many 

 of the Helicida, for the purpose of the notch is to provide a minute 

 orifice, circular in shape, formed in part by the lobes of the clausium, 

 and completed by the curve of the plica-lunata.^ 



This remarkable type of clausium has been noticed only in Alopia^ 

 Clausiliasira {=Marpessa), and in one species oi Herilla — Clausilia 

 dacica Friv. It is probable also, with the exception of Triloba, which 

 I have not up to the present had an opportunity of examining, that 

 the other type, the " entire " clausium, will be found to obtain 

 throughout all other sections of the genus. My description and 

 illustration of the most characteristic forms of clausium will shew 

 this more fully. 



EiLOBATE. — Clausium with two lobes, much resembling a point- 

 ing hand, with finger and thumb extended. Restricted to 

 Alopia. Plate 5, figure i. 



Emarginate. — Clausium deeply notched near the base, on the 

 lower or outer margin. Restricted to Clausiliasfra and Herilla. 

 Plate 5, figure 2. 



Reflexed. — Clausium entire, but with the lower margin bent 

 back, giving a folded appearance, as in Siciliaria, Medora and 

 Garnieria. Plate 5, figures 3 and 4. 



Inflexed. — Clausium bent inwards, and the base raised in the 

 form of an arch, as in SiereophcBdusa and Alinda. Plate 5, 

 figures 7 and 8. 



Rostrate. — Clausium narrowing towards the base, which is thick- 

 ened and has the appearance of a beak, as in Idyla. Plate 5, 

 figure 5. 



Truncate. — Clausium oblong in shape and square at the base, 

 conspicuously truncate at the junction of the spatula with the 

 pedicle, as in Pirostoma and Fapillifera. Plate 5, figure 6. 

 Also in Stereophcediisa. figure 7. 



Clavate. — Clausium narrow at the top, becoming broad or club- 

 shaped at the base, as in Nenia and Bottgeria. Plate 5, 

 : figure 9. 



I Smith and Woodward, On the Nomenclature of the Oral Folds in the Shells oi Clausilia., 

 .\iiii. Mag. Nat. Hibt., March, 1890. 



