MORPHOLOGY OF SHELL. IX 



shell is a character of considerable value in classification. In 

 Jfelicophanta, Acavusand 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 apertuie 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 ZonitidcK. 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, although 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- 

 ralis 00000. Fig. 12 is H. desertorum 123(45). PL 43, fig. 44, is 

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

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

 band split would stand 123345. 



