] 94 Prof. Oiven — On a Jaw of Strophodus. 



parallel ridges which descend to the margin of the crown. There is 

 a like disposition along the fore part of the tooth, but the parallel 

 ridges are formed much nearer the margin, and are much shorter 

 than those behind. The third, fourth, and fifth teeth, a 3-5, closely 

 resemble the second ; the sixth tooth shows the effects of mastication, 

 the threads are worn down to the bottom of the meshes on the most 

 prominent part of the grinding surface, and the meshes are shallower 

 over a greater extent of that surface. In the anterior (seventh, 7 a) tooth 

 abrasion has rendered a still greater proportion of the surface smooth, 

 demonstrating how the ganoin closes the summits of the medullary 

 or vascular canals.^ The unworn meshes of the reticulate ganoin 

 are so minute and deep as to look like pores. The marginal parallel 

 ridges, out of the field of work, of course remain. The six teeth in 

 place preserve the form and dimensions of the innermost : each in 

 succession is moved forward, or toward the symphysis, about 0-004 

 m. m. in advance of the inner tooth. 



The teeth of the row, 6, next in advance are seven in number, 

 and are preserved on both sides of the jaw : they do not show 

 so close a resemblance to each other in size and shape as those 

 of the row behind ; in comparison vdth which they are smaller, 

 more convex in the direction of the length of the grinding sur- 

 face, and the highest part of the convexity is at the middle of the 

 surface : the anterior end of the tooth is narrower than the posterior 

 end, and in a greater degree in the innermost tooth than in the rest ; 

 this character, with the greater longitudinal convexity, gives the 

 appearance of the tooth being bent obliquely lengthwise with the 

 smaller anterior end inclining to the outer side of the jaw. The re- 

 semblance of the tooth, especially of the posterior ones, both in shape 

 and superficies, to a contracted medicinal leech, is close, and accounts 

 for the name given by the quarrymen to the detached fossils. The 

 length of the working surface of the third tooth, & 3, in a straight line is 

 0-031 m. m. ; the breadth of the hind border is 0-013 m. m. ; that of 

 the front border is 0.010 m. m. The posterior parallel linear dis- 

 position of the ganoin is proportionally greater as compared with its 

 minutely reticulate disposition than in the larger teeth of the row 

 behind. In the sixth tooth the summit of the convexity is worn 

 smooth. In the seventh tooth, part of the crown has been broken 

 away, on both sides of the jaw. 



Eight teeth of the row, c, next in advance are preserved on the left 

 side of the jaw : they diminish in greater degree, in size, than do those 

 of the row h compared with a ; they rise higher and more abruptly 

 at the middle of the crown ; the anterior end is more contracted ; the 

 inferiority of size of the innermost as compared with the rest is greater. 

 A low ridge is continued from the summit of the crown to the ends of 

 the tooth, that to the fore end being more marked than the one behind, 

 and the anterior ridges are more prominent in the outer teeth of the 

 row, c, 5, 6, 7, than in the inner teeth, 1-4. The length of the work- 

 ing surface of the fourth tooth, c, 4, is 0-028 m. m. ; the breadth is 

 O-.Oll m. m. 



^ As sliown, in section, magnified, in plate ix. of my ' Odontography.* 



