With reference to the interesting question— What is the homology or essential nature of the ossa marsupialia ? I entirely concur in the opinion first advanced by the able anatomist, M. Laurent,* viz. that they belong to the category of the trochlear ossicles, commonly called sesamoid, and are developed in the tendon of the external oblique which forms the menial pillar of the abdominal ring, as the pa tella is developed in the tendon of the rectus femoris. I had arrived at this conclusion from independent researches, and unaware of any prior announcement of this view when I dis cussed the question before the Zoological So ciety in 1835.t I cannot, however, participate in the opinion of M. Laurent and the celebrated De Blainville, that the marsupial bones are superadded to the abdominal muscles to aid in an unusually energetic compression required to expel the uterine fcetus. It is not in the females of those animals which give birth to the smallest foetuses that we ought to find auxiliary parts for increasing the power of the muscles engaged in parturition. l'he bones in question arc, moreover, equally developed in both sexes : and they are so situated and attached that they add to the power of the muscles which wind round them, and not of those implanted in diem. They are not, however, merely sub servient to add force to the action of the " cre inasteres," but give origin to a great proportion of the so-called " pyramidales." The osteogenesis of the marsupial pelvis de rives some extrinsic interest from the not yet forgotten speculations which have been broached regarding the analogies of the marsupial bones. These have been conjectured to exist in many of the placental Mammalia, with a certain la titude of altered place and form, disguised, e. g. as the bone of the penis in the Carnivora, or appearing as the supplemental ossicles of the acetabulum, which exist in the young of many of the Rodentia. In the os innomina turn of the immature Potoroo the curved pris matic ilium contributes to form, by the outer part of its base, the upper or anterior third of the acetabulum; the rest of the circumference of this cavity is completed by the isehilon and pubis, excepting a small part of the under or mesial margin, which is formed by a distinct ossicle or epiphysis of the ilium, (a, fig. 110,) analogous to that de scribed by Geoffroy St.
Ililaireas the rudimen tal marsupial bone in the rabbit. Now here there is a co-existing marsupial bone : but besides the five sepa rate bones just men tioned, there is a sixth distinct triangular ossi cle, which is wedged into the posterior in terspace of the ischio pubic symphysis. How easy were it to suggest that this single sym metrical bone may be the representative of the os penis removed from the glans to the root of the intromittent organ I 1 regard it as a mere epiphysis of the ischium? The circumference of the acetabulum is always interrupted by a deep notch opposite the obturator foramen, which is traversed by a ligamentous bridge, and gives passage to the vessels of the Ilarderian gland lodged in the wide and deep acetabular fossa.
The genial like the marsupial bone is essen tially an ossification of the fibrous or sclerous tissue.
The femur is a straight, or nearly straight, long, cylindrical bone, having a hemispherical head supported on a very short neck, espe cially in the Petaurists, and situated here almost in the axis of the shaft, above and between the two trochanters, which are nearly of equal size. In the Kangaroos and Potoroos
the head of the thigh-bone is turned more inwards, and the outer or greater trochatitcr rises above it. In other Marsupials the great trochanter is less developed. In most of the species a strong ridge is continued downwards to within a short distance from the trochanter, and this ridge is so produced at the lower part in the Wombat as almost to merit the name of a third trochanter. In the Wombat and Koala there is no depression for a ligamentum teres. The shaft of the bone presents no liner asperse.
The canal for the nutrient artery commences at the upper third and posterior part of the bone in the Koala, and extends downwards, contrariwise to that in most other marsupial and placental Mammalia.
At the distal extremity of the femur the external condyle is the largest, the internal rather the longest. The intermediate anterior groove for the patella is well marked in the Perameles, where the patella is fully developed, but is broad and very shallow in the Phalan gers and Dasyures, where the tendon of the rectos muscle is merely thickened or offers only a few irregular specks of ossification; and the corresponding surface in the Petaurists, Wombat, and Koala is almost plane from side to side; in these Marsupials and in the BIyrme cobius the patella is wanting. I find a distinct but small bony patella in the Mao-opus Ben. ncttii. There is a sesamoid bone above and behind the external condyle of the femur in the Myrinecobius and some other Marsupials.
In the besides the two crucial ligaments continued from the posterior angles or cresses of the semilunar cartilages—one to the outer side of the inner condyle, the other to the interspace of the condyles—there is a strong ligament which passes from the anterior part of the tibial protuberance backwards to the inner side of the fibular condyle, and a second continued from the same point along the outer margin of the outer semilunar car tilage to the head of the tibia.
The tibia (a,fig.111) presents the usual dis position of the articular surface for the condyles of the femur, but in some genera, as the Wombat and Koala, the outer articular surface is continu ous with that for the head of the fibula. In the Kangaroos and Potoroos the anterior part of the head is much produced, and in the young animal its ossification connnences by a centre distinct from the ordinary proximal epiphysis of the bone. A strong ridge is continued down from this protuberance for about one sixth the length of the tibia. In the Koala a strong tuberosity projects from the anterior part of the tibia at the junction of the upper with the middle third. In this species and in the Wombat, as also in the Opossums, Dasyures, Phalangers, and Petaurists, the ;haft of the tibia is somewhat compressed and twisted ; but in the Kangaroos, Potoroos, and Pera meles the tibia is prismatic above and sub cylindrical below. The internal malleolus is very slightly produced in any Marsupial, but most so in the Wombat.