The Didelphia

In the Didelphia, the "odontoid process" early becomes completely anchylosed with the body of the second vertebra; and, usually, all the cervical ribs speedily lose their distinctness, as in Mammals in general.

The coracoid is reduced to a mere process of the scapula and does not come near the sternum. There is no epicoracoid, such as exists in the Ornithodelphia. There is no T-shaped interclavicle, but the clavicles, which are always present (except in Perameles) articulate with the manubrium of the sternum, in the same way as in ordinary Mammalia. The floors of the acetabula are completely ossified, and consequenly are imperforate in the dry skeleton. The cochlea is coiled upon itself.

There is a shallow cloaca, the sphineter muscle being common to the urinary and genital apertures, but there is no such urogenital chamber as in the Monotremata. The ureters open directly into the bladder.

In the male, the urogenital part of the urethra, and that which traverses the penis, form one continuous canal, which opens outward only at the extremity of the penis.

In the female, the vaginal is perfectly distinct from the urinary passage. The mouths of the Fallopian tubes are fimbriated, and the ova are not larger than those of the Monodelphia.

The mammary glands are provided with long teats.
In all the preceding characters the Didelphia agree with the Monodelphia, and differ from the Ornithodelphia.

But they agree with the Ornithodelphia, and differ from the Monodelphia in possessing either bones or cartilages, attached to the pubes, in the position of the so-called marsupial bones of the Ornithodelphia.

Again, the brain, the cerebral hemispheres of which may or may not have a convoluted surface, is provided with a very small corpus callosum, and a large anterior commissure. The hippocampal sulcus is prolonged forward over the corpus callosum.

The crura of the corpus cavernosum of the penis are not fixed to the ischium.

The embryo does not become connected with the parent by villi developed from the allantois, and it is born in a very imperfect condition.

Certain characters are peculiar to the Didelphia. Thus, the testes of the male pass into a scrotum, which is suspended in front of the penis. In the female, the cremaster muscle is largely developed, and spreads over the surface of the mammary gland, which it compresses, so as to drive the milk out of the projecting teat. There is no fossa ovalis on the right side of the septum of the auricles. Very generally, though not invariably, the Didelphia possess what is termed a marsupial pouch, which is a sort of bag, formed by a fold of the integument of the abdomen, into which muscular fibres of the panniculus carnosus extend. These support the ventral wall of the pouch, and are capable of closing its mouth, which may be directed either forward or backward. The mammary glanda lie in the dorsal wall of this pouch, into which the teats project.

There is no direct communication between the female generative organs and the pouch; but the minute young are transported, in the blind and imperfect state in which they are born, into the interior of the marsupium, and each becomes attached to a nipple, which exactly fills its mouth. To this it remains attached for a considerable period, the milk being forced down its throat by the contraction of the cremaster muscle. The danger of suffocation is averted by the elongated and conical form of the upper extremity of the larynx, which is embraced by the soft palate, as in the Cetacea; and thus respiration goes on freely, while the milk passes, on each side of the laryngeal cone, into the oesophagus.

It very commonly happens among the Didelphia that the two long vaginae are bent upon themselves, their proximal ends becoming applied together and dilated, and these dilated portions not unfrequently communicate. Another very general peculiarity of the Didelphia is the inflection of the lower margin of the angle of the mandible inward into a strong horizontal process. In the genus Tarsipes, however, this process is absent.

There are further anatomical characters which are well worthy of notice, though they are not so important as the foregoing.

The integument is always furry, never spiny or scaly, nor provided with dermal scutes. The pinna of the external ear is well developed. In the skull the carotid arteries pierce the basisphenoid to enter the cranial cavity. The tympanic cavity is in front, bounded by the alisphenoid; and, very generally, the jugal furnishes part of the articular surface for the mandible.

Many of the cranial sutures, especially in the occipital region, persist throughout life; and the squamosal, the united periotic ossifications, and the tympanic bones remain distinct from one another.

The jaws are always provided with true teeth; and, usually, these teeth are readily distinguished into incisors, canines, false molars, and true molars. The canines, however, are absent in some genera, either in both jaws or in the mandible. There are usually four true molar teeth, and, as Prof. Flower has recently discovered, only one grinder succeeds another vertically. It represents the last premolar. The molars never possess a complex structure.

No didelphous mammal has three incisor teeth upon each side above and below; and none but Phascolomys has an equal number of incisors in each jaw, the number of the upper being usually in excess of that of the lower jaw.

The number of the dorso-lumbar vertebrae is almost always nineteen; and, of these, six are usually dorsal. The atlas is generally incompletely ossified in the ventral median line. The manus usually possesses five digits, but in Perameles and Chaeropus the outer digits become rudimentary.

The fibula is always complete at its distal end. In some cases it becomes anchylosed with the tibia, while in the Wombat (Phascolomys), the Phalangers (Phalangistidae), and the Opossums (Didelphiae), it is not only free, but is capable of a rotatory movement upon the tibia, similar to the movement of pronation and supination of the radius upon the ulna in Man. The rotation of the fibula toward the ventral side of the tibia is effected by a muscle which, in great measure, occupies the place of the interosseous ligament, and is analogous to the pronator quadratus in the fore-limb. This muscle is antagonized by the extensors of the digits, so far as they arise from the fibula.

The digits of the pes vary remarkably in their form and relative development among the Marsupialia; the different subdivisions of the order being very well distinguished by the modifications of the hind-foot.

Thus in the especially carnivorous Marsupials-the Didelphidae, of America, and the Dasyuridae, of the Australian province-the second and third digits of the pes are not united together by the integument. In the Didelphidae, the hallux is nailless, but large and opposable, so as to convert the pes into a prehensile organ like that of many Primates; in the Dasyuridae, on the other hand, the hallux is rudimentary or absent. In all the other marsupials, the second and third digits of the pes are syndactyle, or united together by integument. In the Wombat, the fourth toe is bound together with the other two, and the small hallux is devoid of a nail. In the Phalangers, only the second and third toes are syndactyle, and they are slender, compared with the other digits, while the hallux is well developed and opposable. In the Peramelidae (Bandicoots) and Macropodidae (Kangaroos), the metatarsus is much elongated, and the second and third digits united and slender, while the fourth toe is very large. The hallux is reduced to its metatarsal bone in the Peramelidae, and the fifth digit is small or rudimentary. In the Kangaroos, the hallux disappears altogether, but the fifth digit remains well developed, though not so large as the fourth.

There is a great range of variation in the characters of the brain. The carnivorous Marsupials (Didelphys, Dasyurus, Thylacinus) exhibit the lowest type of cerebral structure, the olfactory lobes being very large and completely exposed, while the cerebral hemispheres are comparatively small and quite smooth. In the Kangaroos, on the other hand, the cerebral hemispheres present numerous convolutions and are much larger in proportion to the olfactory lobes, which they cover.

The stomach may be simple, as in most Marsupialia, or provided with a cardiac gland (Phascolarctos, Phascolomys). In the Kangaroos, it becomes immensely elongated, with longitudinal muscular bands and transverse sacculations, so that it resembles the human colon. The caecum, which is large in the Kangaroos, but absent in the Dasyuridae, is provided, in the Wombat, with a vermiform appendix like that of Man.

The liver always possesses a gall-bladder. There are two venae cavae superiores, and they receive the venae azygos of their respective sides. The tricuspid valve in the heart is membranous. There is no inferior mesenteric artery, and the external and internal iliacs arise separately from the aorta.

There are no vesiculae seminales, and the glans penis is bifurcated in many species. The marsupial pouch is absent in some Opossums and Dasyuridae. When it is present, its mouth is usually directed forward, but in Thylacinus and in some Peramelidae it looks backward. In Thylacinus also the "marsupial bones" remain cartilaginous. The condition of the foetus is known only in the Kangaroos, and further observations on the embryology of the Didelphia are much needed. The foetus is said to possess a large umbilical sac, the vessels of which extend on to the plaited chorion; and a small allantois; and to be devoid of a thymus gland.

The Didelphia are at present confined to the Australian and the Austro-Columbian provinces, some few species stretching beyond the borders of the latter into the northern parts of North America. The Didelphidae alone are found in Austro- Columbia, all the other groups being Australian.

Gigantic, Kangaroo-like, or Phalangistic, forms (Nototherium, Diprotodon, Thylacoleo), have been found in post-tertiary deposits and caves in Australia. In Europe, Didelphidae occur in Eocene strata; Didelphidae, Dasyuridae, and Macropodidae (Phascolotherium, Amphitherium, Plagiaulax), in middle Mesozoic rocks; and Macropodidae(?) (Microlestes) in the Trias.