250 MARTIN F. WOODWARD. 



than the above, and shows a well-marked hook at its 

 extremity, and a small cusp a little below this; this cusp is 

 still visible on the twenty-seventh (fig-. 40) but completely 

 disappears on the succeeding teeth, which have the form of 

 long massive hooks (figs. 41 and 42), and constitute the 

 largest teeth on the radula. After the thirtieth, the teeth, 

 while still remaining long, become much slighter (fig. 42), 

 and soon (about the thirty-seventh) begin to show signs of 

 the development of two additional cusps (fig. 43), which 

 attain their full development on the forty-ninth tooth (fig. 

 44). The teeth have now the form of long delicate sickles, 

 the free end of which exhibits two deep notches. The last 

 one or two hooked teeth are somewhat shorter than the earlier 

 ones, and thus lead to the distinctly shorter brush teeth. 



I have somewhat arbitrarily drawn the line separating the 

 hooked and the brush teeth between the forty-first and forty- 

 second tooth, thus making seventeen hooked teeth. 



The Brush Teeth. — The forty-second tooth at first sight 

 does not appear to differ materially from the forty-first, but a 

 more careful examination shows that it possesses on either 

 side on a level with the lowest cusp two minute bristles (fig. 

 45). On the next tooth these bristles are longer, and one or 

 two more are appearing (fig. 45) ; and if we examine the 

 feature as we pass outward in the row of teeth (figs. 47 — 

 52) we find that the bristles steadily increase in number and 

 length, until by the forty-ninth tooth they form a consider- 

 able brush reaching to the free end of the tooth. Thus it will 

 be seen that it is impossible to separate the forty- second 

 tooth from the true brush teeth; and although it is more 

 closely approximate in general appearance to the hooked 

 teeth, yet, in the presence of the two minute hairs on either 

 side, it already shows the essential feature of the brush 

 teeth. 



The brush teeth are sixty-three in number, and they form 

 the most characteristic feature of the radula of Pleuroto- 

 maria. 



A tooth taken from the middle of this series (fig. 50) show§ 



