Professor Owen’s Report on the Missourium. 59 
the molar teeth might impress the more cautious paleontologist 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 more 
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 development; 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 giganteum 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 
