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TEETH OF THE VERTEBRATA. 395
considered of sufficient value by some to give the Dldelphia a rank equal
to that of the Prototheria, and they consequently make three primary
divisions of the class Ornithodelphia, Didelphia, and Monoddphla, after
De Blainville. If this were associated with any other characters of
structural importance it would be quite sufficient, but since it is not,
and in view of its unreliability and inconstancy in the lower Vertebrata,
I am not disposed to regard it as equal in value to the strong structural
characters by which the Prototheria are deffiied.
The subdivision of the Monodelphia is not an easy matter, if indeed
any important divisions further than the separation of the mutilate
series can be made. It is convenient, however, to adopt the classifica-
tion of Lamarck, and divide them into three series, as follows : the
mutilate series, the ungulate series, and the unguiculate series. The
first of these includes the Cetucea, or whales, and the Sirenia, or sea-
cows. The only character by means of which they are associated
is the absence of hind limbs and the loss of the articular processes of
the bones of the manus. The Cetacea form a i^erfectly natural and
homogeneous group, and are entitled to a wide separation from all
other ^Mammalia. We at present know very little concerning their
develo])ment or ancestry, further than that their Eocene representative,
Zctiglodon, resembled the ordinary monodelphous type more than does
any other member of the order. They are undoubtedly a very old and
distinct group, and it would not be at all surprising if they are ultimately
found to have descended directly and independently from the Prototlieria.
The Sirenia, or sea-cows, on the other hand, appear to be simply
modified ungulates that have gradually assumed their present structure
in accordance with their aquatic environment. The jNIiocene genus
(Hulitherium) of this order had distinct hind limbs, and in many ways
resembled the primitive hoofed Mammalia. For this reason it is
probably best to associate them with the ungulate rather than with
the mutilate series, since they dilFer in almost every essential feature
from the Cetacea, except in the loss of the posterior members.
The separation of the two remaining series, ungulate and unguicu-
late, depends entirely upon the distinctions to be drawn between " hoof"
and " claw." If we contrast, for example, two such structures as the
claw of the lion and the hoof of the horse, the distinctions are perfectly
obvious, and we will ex})erience no difficulty in recognizing the differ-
ences ; but if we carefully trace the respective lines of ancestry of these
two forms backward to the Eocene Period, we will find them converging
to such an extent as to involve the hoof-and-claw question in almost
hopeless confusion.
There are, however, two principal lines or stems which have terminated
in the distinctly hoof-bearing mammals on the one hand and the claw-
and nail-bearing on the other. The exact point at which these two lines
converge has not as yet been satisfactorily determined, but it is undoubt-
edly true that they a])proached one another to a remarkable extent in
the early Eocene. The ancestry of the entire ungulate series is indi-
cated by the Taxeopoda of Cope, to whose persistent efforts and schol-
arly researches we are alone indebted for their discovery and description.
The primitive or central stem of this order is the Condylarthra, from
TEETH OF THE VERTEBRATA. 395
considered of sufficient value by some to give the Dldelphia a rank equal
to that of the Prototheria, and they consequently make three primary
divisions of the class Ornithodelphia, Didelphia, and Monoddphla, after
De Blainville. If this were associated with any other characters of
structural importance it would be quite sufficient, but since it is not,
and in view of its unreliability and inconstancy in the lower Vertebrata,
I am not disposed to regard it as equal in value to the strong structural
characters by which the Prototheria are deffiied.
The subdivision of the Monodelphia is not an easy matter, if indeed
any important divisions further than the separation of the mutilate
series can be made. It is convenient, however, to adopt the classifica-
tion of Lamarck, and divide them into three series, as follows : the
mutilate series, the ungulate series, and the unguiculate series. The
first of these includes the Cetucea, or whales, and the Sirenia, or sea-
cows. The only character by means of which they are associated
is the absence of hind limbs and the loss of the articular processes of
the bones of the manus. The Cetacea form a i^erfectly natural and
homogeneous group, and are entitled to a wide separation from all
other ^Mammalia. We at present know very little concerning their
develo])ment or ancestry, further than that their Eocene representative,
Zctiglodon, resembled the ordinary monodelphous type more than does
any other member of the order. They are undoubtedly a very old and
distinct group, and it would not be at all surprising if they are ultimately
found to have descended directly and independently from the Prototlieria.
The Sirenia, or sea-cows, on the other hand, appear to be simply
modified ungulates that have gradually assumed their present structure
in accordance with their aquatic environment. The jNIiocene genus
(Hulitherium) of this order had distinct hind limbs, and in many ways
resembled the primitive hoofed Mammalia. For this reason it is
probably best to associate them with the ungulate rather than with
the mutilate series, since they dilFer in almost every essential feature
from the Cetacea, except in the loss of the posterior members.
The separation of the two remaining series, ungulate and unguicu-
late, depends entirely upon the distinctions to be drawn between " hoof"
and " claw." If we contrast, for example, two such structures as the
claw of the lion and the hoof of the horse, the distinctions are perfectly
obvious, and we will ex})erience no difficulty in recognizing the differ-
ences ; but if we carefully trace the respective lines of ancestry of these
two forms backward to the Eocene Period, we will find them converging
to such an extent as to involve the hoof-and-claw question in almost
hopeless confusion.
There are, however, two principal lines or stems which have terminated
in the distinctly hoof-bearing mammals on the one hand and the claw-
and nail-bearing on the other. The exact point at which these two lines
converge has not as yet been satisfactorily determined, but it is undoubt-
edly true that they a])proached one another to a remarkable extent in
the early Eocene. The ancestry of the entire ungulate series is indi-
cated by the Taxeopoda of Cope, to whose persistent efforts and schol-
arly researches we are alone indebted for their discovery and description.
The primitive or central stem of this order is the Condylarthra, from
