20 OPISTHOBRANCHIATA OF BRAZIL 



one-fourth to one-half the length of the median one, and, in the 

 last few rows may bear very minute serrulations, chiefly upon 

 their outer margins. 



The pleural teeth are rather unifomi in outline, the inner- 

 most and outermost being slightly smaller than the remaining 

 ones, into which they pass in a graded series. The general form 

 of the pleural teeth is shown in PI. I, figs. 5 and 6, in dorsal and 

 lateral view. The base is broadest and thickest at its anterior 

 end, tapering to a rounded posterior one which is produced 

 strongly toward the outer border of the radula. In the posterior 

 portion of the radula the base is broader throughout than in the 

 anterior region. The same lateral prolongation is shown through- 

 out the whole length of each row, being most strongly marked 

 about the middle of each (PI. I, fig. 3). The anterior end of 

 the base is recurved dorsally in a strong, broad hook, projecting 

 upward at an angle of about 45° and bearing two strong unequal 

 cusps, which are about as broad as long near the innermost ends 

 of the row, and increase in length progressively until they become 

 about two and one-half times as long as broad, a proportion 

 reached by the tenth tooth (PI. I, figs, i, 2). The smaller 

 external cusp measures one-half to one-third the length of the 

 larger, and is of the same general form. Both cusps bear 

 typically a varying number of well marked irregular denticles 

 upon their margins, which may be entirely separate, or, as is 

 usually the case, are united at their bases into a thin marginal 

 band. These denticles may be fairly uniform in size and shape 

 (PI. I, fig. 2, 10), or more often, very irregular (PI. I, figs. 

 2, 3). The small external cusp bears a lesser number of denticles, 

 which may be few and small, or large and irregular, often at- 

 taining such a size as to give the cusp the appearance of being 

 divided (PI. I, fig. 3). The dimensions of a typical large 

 pleural tooth are: length of base, 0.288 mm., greatest width of 

 base, 0.108 mm., length of larger cusp, 0.102 mm., length of 

 smaller cusp 0.042 mm. The external pleurae decrease pro- 

 gressively in size outwards, the outermost two or three in many 

 cases being reduced to the base alone, the recurved hooks being 

 undeveloped (PI. I, fig. 4). 



Dobson in the Journal of the Linnaean Society (XV, 1880, 

 p. 159), figures several teeth from Tethys dactylomela which agree 



