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  Section: Anatomy of Vertebrate Animals » The Classification and Organization of the Mammalia
 
 
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The Monodelphia

 
     
 

In the Monodelphia, the os odontoideum very soon becomes anchylosed with the second cervical vertebra, of which it appears merely as the odontoid process; and the cervical ribs early become inseparably united with their vertebrae. The coracoid is reduced to a mere process of the scapula, and there is no epicoracoid similar to that of the Ornithodelphia.

Clavicles may be present or absent. When completely developed they articulate directly, or by the intermediation of more or less modified remains of the sternal end of the coracoid, with the sternum, and not with any interclavicle. The acetabula are imperforate. The pelvis is devoid of marsupial bones; though, in some Carnivora, there are small cartilages in the inner tendons of the external oblique muscle, which have a corresponding form and relations.

The anterior commissure and the corpus callosum, no less than the cerebral hemispheres themselves, vary greatly, the brains of some Edentata very closely approaching those of the Didelphia in respect of the corpus callosum and anterior commissure; while, as regards the hemispheres themselves, they may either be so small as to allow the cerebellum to be completely exposed on the dorsal aspect, or so large as completely to cover it and project beyond it. The external surface of the hemispheres, again, may be either perfectly smooth or extremely convoluted.

The cochlea is coiled spirally. The reproductive and urinary apertures, as a general rule, open quite separately from the rectum. The ureters always open into the bladder. The testes may remain in the abdomen throughout life, or may pass into a scrotal pouch. But, when this scrotum forms a distinct sac, it lies at the sides of, or beliind, the penis, and not in front of it. The cystic urethra is always continuous with that part of the urethra which traverses the penis.

The ova are small, and the mouths of the Fallopian tubes are fimbriated. The vagina is a single tulie, which may, however, be partially divided by a longitudinal partition. The cremaster has no relation to the mammary glands, which are provided with distinct teats.

The allantois is always well developed, and gives rise to a placenta; and the young are born of large size, and active.

The great majority of the Monodelphia, as thus defined, are divisible according to the characters of their placenta into non-deciduata and deciduata.

In the non-deciduata the foetal villi of the placenta are, at birth, simply withdrawn from the uterine fosste, into which they are received, and no part of the maternal substance is thrown off in the form of decidua, or maternal part of the placenta. In the deciduata, on the other hand, the superficial layer of the mucous membrane of the uterus undergoes a special modification, and unites, to a greater or less extent, with the villi developed from the chorion of the foetus; and, at birth, this decidual and maternal part of the placenta is thrown off along with the foetus, the mucous membrane of the uterus of the parent being regenerated during, and after, each pregnancy.

There are, however, two orders of existing monodelphous Mammalia, the nature of the placentation of which is not yet fully made out. One of these is the Sirenia, the placentation of which is unknown. The other is the ill-defined and heterogeneous assemblage called Edentata. Some of the members of this group certainly possess deciduate placentae, while, in others, it appears questionable whether the decidua is, or is not, developed. And, as this group, the Edentata, is decidedly the lowest of the whole division, I shall take it first in order, while the Sirenia are arranged, provisionally, among the Non-deciduata.


 
     
 
 
     



     
 
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