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TEETH OF THE VERTEBRATA. 493

departure from the primitive tritubereular ancestry. In the lower jaw
the molars are lophodont, while in the upper they have a single cres-
cent of moderate perfection. Owing to their near relationship with the
Toxeopoda, it is highly probable that their teeth represent an extreme
modification of the tritubereular pattern, but of the different steps in
their production lack of space i)revents me from speaking here. 1 nuist
refer the reader to the papers of Profs. Cope and Marsh for a more
complete description of the dentition of this order.

Teeth of the Marsupials.
I have indicated on a preceding page that this division of the Mam-
malia is sharply defined from the monodelphs by the circumstance that
no connections are formed between the foetal envelopes and the walls of
the uterine cavity during gestation, so that no placenta is developed.
They are therefore known as the implacental division of the Eutheiia ;
they are likewise known as the Didelphia and Marmpialia. The young
are born in an exceedingly helpless and imperfect condition, and are
transferred to the pouch or marsupium of the mother, where, by a special
arrangement, the nourishment is forced into their months until such time
as they are enabled to help themselves.
In the majority of the lower Verlebrata very little devektpment of
the young takes place in the body-cavity of the mother ; the ovum is
relatively large, by reason of the addition of an abundant supply of pab-
ulum suflticient to nourish the embryo until the later stages of develop-
ment are reached. It has been recently ascertained that the monotremes
reproduce in the same way; that is, they lay eggs like birds and rep-
tiles, which are hatched in a similar manner. The whole plan of devel-
opment moreover, is like that of the bird (mesoblastic)—a condition
which would be reasonably suggested by a study of their reproductive
system.
As the monotremes furnish the connecting link between the higher
mammal and the reptile, so do the marsupials, as far as reproduction is
concerned, afibrd a transitional stage between the monotremes and the
monodelphs. For this reason we Avould naturally be led to look for
primitive and transitional characters in their teeth. Unfortunately,
these organs do not in many particulars go beyond the lowest forms of
the monodelphs sufficiently to give us any clear insight into the inter-
mediate structures and patterns which must have preceded the (li])hyo-
dont monodelph dentition ; still, some of the earliest representatives of
mammalian existence AA'hich have been referred to in this group possess
a greater number of heterodont molar and premolar teeth than any
known mammal.
In the small living marsupial genus Myr'wecobivs the dental formula
is I. ^, C. \, Pm. ^I. = 54. The incisors are small, subconic teeth,
I, f
implanted in the premaxillary bones above, and followed by the canines,
which have the usual laniary form. The premolars have laterally-com-
pressed, unicuspid crowns, and are implanted by two roots. The molars
exceed in number those of any other marsupial, reaching the unusual
number of six in each jaw. Owing to the imperfect descriptions of