156 G. H. PARKER 
(07) and are apparently characteristic not only for species but 
for larger groups of gastropods. In the majority of species thus 
far examined, the pedal waves course forward over the foot, thus 
agreeing in direction with the animal’s locomotion. Vlés has 
appropriately designated this type of movement as the dzrect type 
and has given the following gastropods as examples; the pulmo- 
nates (including Onchidium), Aplysia, Aeolis, Doris, Haliotis, 
Trochus, Cyclostoma, and certain small species of Littorina. I 
can confirm this statement for such of these molluscs as I have 
examined, namely, many pulmonates, including Onchidium, and 
I can add to this list Crepidula fornicata. In other gastropods 
the waves pass over the foot from anterior to posterior and this 
type has been designated by Vlés as retrograde. As examples he 
has given Acanthochites fascicularis, Littorina littorea, and L. 
rudis. Besides confirming Vlés’ observation on Littorina lit- 
torea, I can add to this list Dolabrifera virens Verrill, Tectarius 
nodulosus Gmel., Nerita tessellata Gmel., and Chiton tuber- 
culatus Linn. According to the observations of Jordan (01, 
p. 99) Aplysia belongs under this head and not under that of the 
direct type as given by Vleés. 
In both chief types of movement several subtypes can be dis- 
tinguished as determined by the lateral extent of the pedal waves. 
In some gastropods each wave extends over the functional width 
of the foot and thus the foot is occupied by only a single series 
of waves. This subtype has been termed by Vlés monotazic, 
and is exemplified by the pulmonates and chitons. In addition 
to these gastropods, Dolabrifera virens also has a monotaxic 
wave. In other gastropods the foot is functionally or even struc- 
turally divided along the median plane and exhibits a double sys- 
tem of waves, one right and the other left. This subtype has been 
designated ditaxic by Vlés and is exemplified by Haliotis, Tro- 
chus, and Cyclostoma among the direct types, and by Littorina 
littorea among the retrograde types. Besides confirming Vlés’ 
statement as to Littorina littorea, I can add Tectarius nodulosus 
and Nerita tessellata as ditaxic gastropods. In Tectarius the 
waves on the two sides of the foot usually alternate and they are 
