COLOR PATTERNS. XXI 



The zoned or many-banded patterns are a higher (more 

 evolved) stage; the streaked are lower in the scale; but from 

 the co-existence of all stages in many species, it appears that 

 the evolution of one from the other is a rapid process. 



The transition from longitudinal streaks to spiral bands in 

 these shells is entirely analogous to the transition from longi- 

 tudinal to transverse stripes in mammals. It is a common 

 phenomenon in mollusks, and examples can readily be found 

 in most faunas, exactly parallel to the process in Achatinella. 

 Cf. Drymceus, Amphidromus, Helicostyla. 



Spiral bands or lines, resulting from the evolution of stripes, 

 as described above, are indefinite and variable in number and 

 position. 



The four-band pattern. The black-brown bands, which 

 stain the prismatic layer of the shell and often appear as spots 

 on the lip, have perfectly definite positions in Achatinellas- 

 trum, Bulimella, and a few Partulinas, when they occur at 

 all, and are evidently homologous throughout these groups. 

 See plate 38. These bands are four in number: 1, sub- 

 sutural; 2 and 3, above and below the periphery; and 4 

 around the columella. All are developed in pi. 38, figs. 4&, 

 6, 6a, 14. Often only bands 1 and 4 are developed, figs. 1, 

 la, 2-4, etc. Other forms may have bands 2 and 3, or various 

 other combinations. The bands of the four-banded or "tetra- 

 taeniate" system may be expressed by a formula, such as is 

 used for the five-banded or so-called pentatseniate Helices. 

 Thus pi. 38, fig. 6 has the formula 1234. Fig. 7, 0000. Fig. 

 7c, 0230. Fig. 18a, 1(234). Fig. 19c, 1(23)0. In Acha- 

 tinellastrum a particularly common formula is 1000. As in the 

 pentataeniate Helices of Europe, the bands vary through the 

 several possible combinations in presence or absence, and in 

 width when present; they are sometimes split, or confluent; 

 but their positions are invariable. 



In inheritance, bands of this system are entirely independ- 

 ent of the variable or indefinitely-placed bands. They may 

 exist in combination with any stage of the other pattern, 

 from plain or streaked to many-banded; and even in melan- 

 istic shells they may be apparent as bands of slightly different 

 luster, visible only in a certain light. 



