356 MR. OWEN ON THE GENERATION OF THE MARSUPIAL ANIMALS. 



most Other mediastina in the human body, consisted of two laminae combined. Of 

 these, each vag-ina furnished one ; for each had its own constrictor*." 



To understand the relations which the female sexual apparatus of the Ornithorhyn- 

 chus and Echidna bear to those of the ordinary Mammalia, it becomes necessary to 

 consider through what families the human or concentrated form of the apparatus 

 degenerates towards the oviparous structure, by the second mode of deviation. The 

 first step in this descent is presented in the Sloths. The uterus is here of a simple, 

 elongated, undivided form ; but the distinction between the vagina and uterus by an 

 OS tincoe is lost, and so far it resembles an oviduct of a reptile. The uterus presents 

 a similar form in the Nine-banded Armadillo (Plate VI. fig. 4.), but in the Weasel- 

 headed Armadillo the angles are slightly elongated. 



Moreover, in these as well as the more decided Edentate genera, as Manis and 

 Myrmecophaga, the urethro-sexual canal is formed, as in Tortoises, by a continuation 

 of the urethra or urinary bladder, into which the genital tube opens by a small ori- 

 fice, (but in the Sloth, in the virgin state, by two small orifices,) just as the urethra 

 communicates with the vagina in other Mammalia. The vaginal portion of the tube 

 is indicated by the thinness of the parietes in the lower or distal half, by the smoother 

 and less villous structure of the lining membrane, and its disposition in regular 

 longitudinal rugae ; but the change is gradual, and the exact extent of the uterus 

 is not marked by any constriction. 



Now the Ornithorhynchus and Echidna, while they present a complete division of 

 the efferent portion of their generative apparatus like the other Marsupiata, maintain, 

 in the composition of each lateral moiety and in its mode of termination, their affinity 

 to the Edentate order, in which Cuvier has placed them ; and thus, by combining 

 those characters'^ of the oviparous type of the generative system which separately 

 present themselves in other Mammalia, there results that affinity to the structure of 

 the same parts in the Reptilia which has led to the supposition of their forming a 

 distinct class of animals. The complex scapular apparatus of the Monotremata, the 

 mandibles of the Ornithorhynchus, and the structure of the male intromittent organ, 

 which, though perforated by a complete canal, is adapted to transmit the seminal 

 fluid only, form additional deviations from the mammiferous structure. But the single 

 cloacal outlet, the double superior cava, and the absence of the inferior mesenteric 

 artery are approximations to the oviparous type participated by them in common 

 with the other Marsupiata ; and the whole may be regarded as an aberrant group 

 of Mammalia characterized by an ovo-viviparous generation. 



A few remarks remain to be added respecting the vaginae of the Kangaroo. 



separated into two canals, each receiving its corresponding os tincse, as in the Opossum, In the other case the 

 uterus was divided hoth externally and internally into two lateral compartments, but the vaginal septum com- 

 menced a short distance below the uterine orifices, as in Dr. Purcell's case. — See Breschet's Repertoire 

 d'Anatomie, torn. v. p. 99. 



♦ Philosophical Transactions, vol. Ixiv. p. 478. 



