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) wliich 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 

 MazzarelU and ZuccarcU (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 nmi. 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 shghtly 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 fom" 

 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. Foiu* 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 



