50 THE NAUTILUS. 



the same, following each other towards the anterior, but in some 

 instances do not reach the latter. The number of simultaneous 

 waves is various in the several species, genera and groups, but rather 

 constant in one species ; they also vary in extent, and may be faster 

 or slower, regular or irregular, very distinct to more or less obscure. 

 In most species, they are confined to a median zone of the sole, nar- 

 rower or wider, corresponding with the so-called locomotive disk. 

 In some species, resp. groups, that disk is marked off from the 

 marginal zones by a more or less marked line, more or less distinct 

 in alcoholic specimens. But in some the zones are not noticeable on 

 the dead body, while in the living, creeping animal, the waves are 

 very distinct and sharply restricted to the median zone. This was 

 especially noticed in Polygyra (Stenotrema, Triodopsis + Mesodon}, 

 and in Helix hortensis, for which the existence of a disk has been 

 denied ; also in some of the Zonitidce, A griolimax, Vertigo, Succinea. 

 But there are noticeable differences, as will be stated later on. 



In a few snails, of various groups, the waves extend over the 

 whole width of the sole, e. g., in Oircinaria concava Say, Vallonia 

 sp., Bifidaria armifera Say. There are no marginal zones in these, 

 so far as I was able to see, and it is probable that the muscles of the 

 sole are different, comparatively wider than the others. 



In a number of others, no waves could be seen, and no differenti- 

 ated longitudinal zones. Such were : part of the Zonitida, such as 

 Gastrodonta ligera, Zoniloides nitidus, arboreus, minusculus, all 

 Patula, Helicodiscus, Philomycus. Moreover, it appears that at 

 least in the Zonitida cited, the sole is of a formation different from 

 that of, e. g., the Polygyra;; it seems that an additional layer of 

 tissue is superposed on the under surface of the foot. The surface 

 has a different appearance, the sole seems thicker, and the double 

 lateral lines, above the margins of the foot appear to point to the 

 same conclusion. Unfortunately, I had no time to make exact ana- 

 tomical and histological examination of these parts, but some 

 anatomist may take the subject up. 



It is interesting and significant that such differences are found 

 among our Zonitidce. It has been pointed out, long ago, by some 

 scientists, that the family includes some widely different forms or 

 types, with all similarity of the shells, and even the radula, etc. 

 And it will be noted that, e. g., Gastrodonta and Zonitoides, which 

 were denoted as showing no locomotive waves, no longitudinal zones, 



