The nudibranch family Dironidae. 523 
occupied by the radula and its cartilages. In Dirona albolineata 
(Pl. 31 Fig. 18) this ridge is much less strongly developed, though 
the same muscle relations exist. 
The dorsal margin of the wing is straight, the posterior and 
ventral ones curved. On the anterior, inner surface is borne a broad, 
thick, shield-like plate (Pl. 31 Figs. 5, 6, 17, 18, m), continuous with 
the thickened, inner surface behind and above, and refleeted across 
the front margin of the mandible This masticatory plate is 
strenethened in its attachment by three radiate thickenings of the 
mandible which are best seen by transmitted light and are shown 
in the outline drawing of Fig. 7, 7,2,3 of Pl. 31, which represents 
the outer surface of the right mandible of Dirona pieta. The upper- 
most of these extends along the dorsal border of the wing, the 
lower extends obliquely from the lower portion of the wing upward 
toward the hinge region, while the anterior one is a thickening 
along the masticatory surface itself. It is to be understood that 
these do not appear as elevations upon the surface alone, but are 
denser, more compact regions of the mandible substance, which are 
evident by transmitted licht. The three thickenings center in the 
resion of the fulerum of the mandible, which is especially thick and 
massive, and bears the hinge uniting the lateral halves together. 
This hinge is made up of two portions, an anterior ligament, opti- 
cally distinet from the mandible substance (A), and a posterior, 
triangular chitinous band (c), which is a direct continuation of the 
mandible substance on either side. The upper surface of this chitin 
band is deeply grooved, presenting a V shape in cross section as 
shown in the cut surface c of Figs. 6 and 18 of Pl. 31. This 
groove is continued downward on the external face and merges into 
the dorsal surface of the mandible. The posterior wall of this 
groove is prolonged into a thin expansion extending obliquely upward 
aud backward. The plate thus formed furnishes a support for the 
roof of the anterior pharyngeal cavity, and above it is situated the 
flat transverse band of muscle which opens the mandibles. 
The masticatory surface is slightly hollowed in its central 
portion and roughened irregularly. Other than this it is entirely 
smooth, with no trace of a denticle armature so common in other 
Aeolids. The only mandible in any way resembling that of Dirona 
is tliat described by VAyssıEreE (1903) for Madrella aurantiaca, which 
species differs widely from Dirona in all other respects. 
The radula, radula sack, and rotella are situated in the 
