INTERNAL ANATOMY. 317 
upward, with a thinner median prolongation shading off into the flattened pos- 
terior end of the tooth (Plate 6, fig. 6-8). Occasionally an evidence of a short 
bluntly pointed compressed median hook appears at the anterior end, (Plate 6, 
fig. 9) which represents the median tooth from the fifty ninth row, but no lateral 
denticulations, such as those figured by Bergh (1905) for D. rumphii [= scapula} 
are to be found. The structure is likewise much simpler than that found by 
Mazzarelli and Zuccardi (1890) in D. hasseltii, D. teremidi, and D. tongana 
[= ecaudata], and by Farran (1905) in D. scapula, the only other species of 
Dolabella whose radulas have been described and figured. In length the median 
tooth ranges from 0.192 mm. to 0.210 mm., in width at the anterior end 0.012 
mm. to 0.018 mm., at the posterior end from 0.063 mm. to 0.084 mm. 
The lateral teeth are simple strongly compressed hooks borne vertically 
upon the surface of the radula-membrane. In general they differ but little 
in size and form, the first three or four inner laterals being slightly smaller 
than the remainder, while the outermost four or five decrease rapidly in size. 
A typical lateral tooth from the posterior portion of the radula is shown in 
side view in Plate 6, fig. 2. The base, a-b, is straight and narrow, averaging 
0.246 mm. in length, and 0.012 mm. in maximum width. Its posterior end, 
b, slopes rapidly downward to the surface of the cuticular membrane, to which 
the teeth are attached. From the anterior end rises the strong but narrow 
hook, a-c, its anterior margin thickened and supported behind by a thinner 
median part. The tip is blunt and rounded, though occasionally slightly bifid 
tips may be found. The vertical height of the hook above the bottom base line 
averages 0.336 mm., and its thickness is the same as that of the base, 0.012 mm. 
Here and there abnormal teeth may be seen, apparently formed by the fusion 
of two normal ones, the united hooks being more or less separated toward the 
tips. Such an abnormal form is usually repeated at the same point in successive 
rows of the radula throughout the whole or the greater portion of its extent, 
being caused by some malformation of a definite group of odontoblast cells 
at the bottom of the radula-sheath, each successive tooth produced by them being 
similarly deformed. An instance of this (Plate 5, fig. 3) represents the four 
innermost teeth of the thirtieth row. The hook of the fourth of these worn 
teeth is broad and deeply bifid, as is also the base. Four teeth from the corre- 
sponding position in the fifty sixth row are given in Plate 5, fig. 5. The same 
bifid condition is found in the fourth tooth here, the division of the hook, how- 
ever, being nearly complete. These teeth are from within the radula-sheath 
and have not yet been exposed to wear, though the tip of the innermost one has 
