MORPHOLOGY OF SHELL. IX 



^-shell is a character of considerable value in classification. In 

 Helicophanta, Acavus and their allies it is very large, sometimes one- 

 third the diameter of the adult shell, and its junction with the post- 

 embryonic growth is distinctly marked. In Polygyra it is very 

 -small and indistinct. In Camcena and allied groups it is of medium 

 \size. Some genera have the embryonic shell sculptured, as Ano- 

 glypta, Chloritis, certain species of Helicigona and Pleurodonte, but 

 it is usually smooth and polished. 



The aperture is usually crescentic, half-round or round, but in 

 keeled species becomes angular, and in those having teeth it is often 

 ear-shaped. The outer lip is expanded, reflexed or thickened within 

 in nearly all the genera, but in some (Sagda, Glyptostoma, etc.) it is 

 simple and sharp as in Zonitidce. Tooth-like processes are fre- 

 quently developed upon the lip and parietal wall, and sometimes 

 these become excessively complex. Usually there are two teeth 

 upon the lip and one upon the body wall ; totally diverse genera 

 having independently evolved this arrangement. In a few groups 

 there are internal plates or septa, far within the mouth. 



The banding of Helices, althongh variable as a specific character, 

 often shows considerable constancy in a genus or subgenus. Thus, 

 in Helix the five-banded plan of coloring is usual. In Helicigona 

 one- or three-banded ; Epiphragmophora is one-banded. The band 

 just above the periphery is the most constant, and may be found in 

 most genera of Belogona. The Epiphallogona have their own 

 band-arrangement, noticed on p. 103. Snails inhabiting dry situa- 

 tions or arid regions, deposit more lime in the shell than those liv- 

 ing in moister places, and there is a strong tendency to split the 

 bands into many narrow lines, as in Euparypha, Helicella, Rhagada, 

 Micrarionta. 



A convenient formula was invented by Georg von Martens many 

 years ago, for the designation of band variations in Helices, espe- 

 cially the five-banded forms. The bands are numbered 1, 2, 3, 4, 

 5, beginning above. The absence of any band is indicated by a 

 cypher ; the coalescence of bands by parenthesis ; and the splitting 

 of a band by repetition of its number. Thus, the specimen shown 

 in fig. 5, of plate 44, is Helix nemoralis, 12345. Fig. 4 is H. nemo- 

 rails 00000. Fig. 12 is H. desertorum 123(45). PI. 43, fig. 44, is 

 H. saulcyi 1(23) 40. A specimen with the bands united to conceal 

 all the ground color would be (12345) ; and one with the third 

 Iband split would stand 123345. 



