26 SHARKS. 



sent such modifications of their common and characteristic type of 

 structure as fits them for very different habits of Hfe and the acqui- 

 sition of different kinds of food. The active, and predatory sharks 

 are here associated with the sluggish omnivorous rays, and the 

 dental system presents every grade of modification from the laniary 

 to the molary type ; the Lamna with its teeth exclusively adapted 

 for holding, piercing, and lacerating, and the Myliohates with its 

 maxillary mosaic pavement of flattened molars forming the two 

 extremes of the series. 



13. The sharks, or Squaloid plagiostomes, with few exceptions, 

 have teeth of a conical, sharp-pointed, more or less compressed form ; 

 sometimes with trenchant or serrate edges and accessary basal den- 

 ticles ; they are arranged along the margin and posterior surface of 

 the jaws in close-set vertical rows, of from three to thirteen teeth in 

 each row, according to the species. The teeth of the contiguous rows 

 in certain genera, as Selache, and Lamna, are parallel with each other, 

 but in Galeus, and Carcharias, they are placed alternately, so that the 

 base of one tooth advances laterally into the interspace of two teeth 

 of the contiguous row, and reciprocally ; but the laterally contiguous 

 teeth are never articulated with each other as in certain rays. In 

 the Scymnus, the median row of teeth crosses the symphysis of the jaw. 

 and their base overlaps the adjoining margins of the contiguous teeth; 

 the lateral teeth have an imbricated arrangement. 



In general the anterior or external tooth only of each row is 

 erect, the rest being recumbent ; the contrast, in this respect, is most 

 marked in the lower maxillary lancet-shaped teeth of the Scymnus, 

 (PI. 4, fig. 3). In La?7iwa, however, the second and third teeth are 

 commonly seen in positions intermediate between those of the erect 

 anterior and the recumbent posterior teeth, (PL 5, fig. 1,) and in the 

 rays where the teeth are much more numerous in each row than in 

 the sharks, they exhibit every gradation between the recumbent, 

 reflected, erect, and porrect positions. It is scarcely necessary to 

 repeat, that although the teeth of the sharks possess greater individual 

 mobility than those of the rays, the recumbent ones cannot, as has 

 been supposed, be voluntarily erected ; these teeth are still in 

 progress of development, and several of them are covered by a re- 



