THE ANATOMY OF PLEUROTOMAEIA BEYRTCHTI. 249 



inclined to the basal membrane, and its antei"ior extremity is 

 much widened. The form of these teeth may be best under- 

 stood by an examination of figs. 32 — 36. 



The Lamellate Teeth. — It will be seen from figs. 32 and 

 36 that the central teeth are not sharply marked off from the 

 more laterally placed lamellate teeth, but pass imperceptibly 

 into them. Thus, owing to its size, the first lamellate tooth 

 might be almost equally well classified with the central 

 teeth. 



There are twenty-one teeth which may be grouped under 

 this head. Though varying in size, they are, on the whole, 

 the smallest teeth in the radula, and present very uniform 

 characters. The free end of each of these teeth is abruptly 

 truncated, and the upper border is generally straight, while 

 the under margin is either convex or angulated, and the base 

 of attachment is small (figs. 32, and 37 A, B, C, B). The 

 lamellate teeth are far more complicated than would appear 

 at first sight, so much so that it is very difficult indeed to 

 gain any idea of their form from a written description. I 

 have therefore thought it better to give several drawings of 

 one of these teeth in diiferent positions (see fig. 37 A, B, C, 

 J)), and to these I must refer the reader who desii-es to obtain 

 an idea of the form of these very characteristic teeth. 



The first five or six of these teeth present a slightly concave 

 free border, and thus approach the central pairs, which they 

 further resemble in the greater development of the outwardly 

 flexed free margin, which evidently represents the last trace 

 of the overlapping lamella of the central teeth. 



As we pass outward the lamellate teeth increase in size, 

 and thus approximate to the hooked teeth (figs. 32 and 38). 



The Hooked Teeth or Uncini. — The gradation between 

 the lamellate and the hooked teeth is completed by the 

 twenty-fifth tooth (fig. 38), which, though only slightly laro-er 

 than the preceding tooth, approximates in form to the more 

 lateral hooked teeth. It will be seen to present a double 

 curvature in its free margin, and a slightly hooked free 

 extremity. The twenty-sixth tooth (fig. 39) is much larger 



