524 F. M. MacFarran, 
extreme upper portion of the space between the mandibles, all the 
lower two-thirds being occupied by a powerful transverse adduetor 
muscle, which binds the mandibles together below. The fibres 
of the lowermost portion of this muscle are directly transverse, 
while the remainder is made up of two distinct sets of fibres, arranged 
separately and crossing each other at a slight angle. The alternate 
contraction of these different muscle fibres would give the masti- 
catory surfaces of the mandible a grinding motion upon each other, 
for which their shape is admirably adapted. 
The short, tubular radula sack is horizontal, the active portion 
of the radula being curved downward in front. The radula is deeply 
erooved in its anterior portion, and bears five transverse rows of 
teeth, the median row occeupying the bottom of the groove, and widely 
separated from the lateral rows, which are borne just within, and 
upon the crest of the groove respectively. 
While the teeth of the two species are of the same general 
form, the teeth of Dörona albolineata are decidedly the larger, as may 
be seen by a comparison of Figs. 1 with 11, 4 with 19, and 10 with 14 
ot Pl. 31, all of which are drawn at the same magnification. The 
average differences in size may also be gathered from the following 
table, though some slight variation is to be expected in different 
regions of the radula. 
Dirona pieta Dirona albolineata 
Median tooth: 
Total length 0,060 mm 0,072 to 0,080 mm 
Length base 0,057 0,048 to 0,050 
First lateral: 
Total length 0,168 0,198 to 0,216 
Length base 0,114 0.135 to 0,198 
Length hook 0,054 0,06 to 0,078 
Second lateral: 
Length base 0,198 0,210 to 0,228 
Heieht hook 0,126 0,132 to 0,168 
The first lateral tooth of Dirona pieta (Pl. 31 Figs. 8, 9, 10) 
bears a series of small denticles, four to seven in number, on its 
inner basal surface, while the similar tooth of Diona albolineata 
(Pl. 31 Fig. 13, 14, 16) bears at most but four such denticles. 
I have been unable to find any trace of the rudimentary denticles 
upon the base of the median tooth as described by Erior (1905). 
