150 Geological Society. 



observes, be referred to two distinct species, though the identity in 

 the molar teeth might impress the more cautious palaeontologist with 

 a strong suspicion of their generic identity ; but the cranium of the 

 male Narwhal, with its unsymmetrical distortion, increased by an 

 enormous tusk, would, it can scarcely be doubted, be referred to a 

 genus of Cetaceans quite distinct from that which the edentulous and 

 more symmetrical skull of the female would be considered to represent. 



In determining the real nature of differences in these extinct 

 animal remains, Mr. Owen says it is necessary to inquire what other 

 modifications are associated with those of the tusks ; — are the m.ore 

 essential parts of the dental system, as the grinding teeth, alike or 

 different in the jaws with tusks and without tusks ? Do the jaws 

 themselves and the other parts of the skeleton offer the modifications 

 of form which usually attend distinction of species ? Above all, are 

 the same characters presumed to distinguish the genera, present in 

 the young as in the adult skulls ? are there, for example, young 

 Mastodons as well as young Tetracaulodons ? 



The youngest of five full-grown Tetracaulodons or male Masto- 

 dons, examined by him, had two molars and half of a third deve- 

 loped in each ramus ; the first or antepenultimate having three trans- 

 verse ridges, each divided into two tubercles ; the second also three 

 bicusped ridges ; and the third two ridges extricated, and two others 

 within the alveolar cavity. In the next jaw in the order of develop- 

 ment, the third ridge of the last molar was extricated ; in the third 

 specimen the antepenultimate grinder had been shed, and the last 

 molar exhibited the same degree of developm.ent ; in the fourth jaw 

 the ultimate molar was fully extricated, exhibiting four bicuspidate 

 ridges and a talon ; and the fifth or oldest Tetracaulodon retained 

 its penultimate but worn grinders, the two anterior ridges of the last 

 molars being a little abraded, and the talon being developed into a 

 pair of small tubercles. 



A series of jaws of female Mastodons (Mastodon proper of Dr. 

 Godman and Dr. Hays) presented the same order of development. 



Having already shown that the molar teeth are identical in number 

 and form in the Mastodon and Tetracaulodon, Mr. Owen proceeds to 

 point out their correspondence in the mode and order of succession. 

 The lower jaws of both present, moreover, those characters by which 

 the Mastodon giganteiim is distinguished from the genus Elephas, 

 namely, by the higher coronoid, the less-rounded angle, the straight 

 inferior margin, the parallel inner alveolar border, and the more pro- 

 duced symphysial extremity. They present, besides, equally the 

 minor characteristic of the sharp process on the inner side of the 

 neck of the condyle, and the ridge continued from the outer side of 

 the neck. Both have an oblong depression on the outside of the 

 coronoid process, but varying in depth in different Tetracaulodons. 

 In both the posterior aperture of the dental canal commences in the 

 same place ; and the inner side of the angle of the jaw is concave, 

 and bounded by an irregular margin, indicating the attachment of 

 the fascia covering the internal pterygoid muscle, the irregularity 

 being stronger in the lower jaws of older individuals. The relative 

 position of the principal anterior outlet of the dental canal is the 



