62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



masticatory is proved by the deeply worn facettes along the lateral sur- 

 face of the crowns on either side. These marks of contact with an 

 opposed paired series are most conspicuous on the first four or five teeth 

 counting from the front, indicating that these remained functional for a 

 considerable period, while those following were just beginning to come 

 into play. From the fact that the front face of the smaller anterior 

 tooth is deeply worn, it is clear that the series has been preserved 

 intact in this direction ever since the creature was alive, and this pol- 

 ished front face cannot be accounted for except as caused by abrasion 

 against hard foreign objects, such as sharks of bottom-feeding habits 

 might encounter. It appears practically certain that the eleven teeth 

 which are still preserved in the Nebraska specimen, and thirteen in 

 the Kansan, were altogether retained within the mouth cavities of their 

 respective owners, and did not protrude relatively further forward than 

 do the anterior series of the modern Cesti'acion. 



The considerations which uphold the views just stated are as follows : 

 (1) In the Kansan specimen the supporting cartilage to which the indi- 

 vidual teeth are attached, and which forms part of the symphysis itself, 

 is continued so far as the anterior (distal) tooth, which at the same 

 time is the most worn of the series ; (2) in addition, the cartilage 

 referred to is studded with numerous small " teeth," or more properly 

 speaking, irregular enamel-covered bosses, tubercles, or patches of den- 

 tine.^ These would not be present along the bases of the median series 

 if the latter had projected freely beyond the mouth region ; (3) if 

 the worn anterior portion of the arch had become so far displaced by 

 successional teeth as to have protruded from the mouth, those teeth 

 which in their turn became functional would exhibit prominent indica- 

 tions of wear ; whereas, in fact all behind the first four or five are but 

 slightly abraded. 



The worn facettes along the sides of the anterior, or as they ma}'^ 

 properly be termed, functional teeth, are not plane, but slightly concave ; 

 and each tooth is bevelled at a slightly independent angle from the rest. 

 The enamel has of course been entirely worn through, so that the 

 underlying dentine tubules are displayed in section. The enamelled 

 prominences which have been referred to as flanking the lateral extremi- 

 ties of the median series have their oral surfaces likewise abraded. 

 Although the enamel is not preserved on any of the coroual apices of 



1 Similar bodies are figured by St. John and Worthen in connection with the 

 lateral series, and interpreted by them as " placoid scales." Pal. 111., Vol. VI. 

 (1875), p. 315. 



