Dasyurus ursinus.—The average diameter of the blood-discs is win : observed extremes of size is and Perameles Ivotis.—The blood of this Mar supial, which was examined while recently drawn from the living animal, and under the same circumstances as that of the two species of Dasyure, presented a still greater number of the granulated blood-discs mixed with others of the ordinary form. The descriptions of such altered blood-discs not only by llewson and Falconer, and in recent times by Professor Wagner, (Ilecker's Literlirische Annalen, Fe bruarheft, 1834,) but also in works in our own language, as in Hodgkin's Translation of Ed wards's Influence of Physical Agents on Life, Appendix, p. 438, 1832, have rendered the fact sufficiently familiar. But the connection of this well-known appearance with the mode of formation or multiplication of the blood par ticles had not before attracted the attention it seemed to deserve; on this subject I have else where remarked : " In some of the granulated blood-discs of the l'erameles the subdivisions producing that appearance were fewer and larger, and were separated by deeper clefts than I had before observed ; they suggested to me the idea that the blood-disc was under going a spontaneous subdivision into smaller vesicles, and, although my observations are not at present sufficiently numerous to warrant the hypothesis that the development of smaller ve sicles within itself is a normal property of the ordinary coloured vesicle or blood-disc, yet the obscurity which still hangs over the origin and reproduction of the blood-discs, and the unex pected constancy of the granulated form in a greater or less proportion of them while recent, and floating in the serum, in different species of animals examined by me, makes me unwilling to suppress any idea naturally arising out of such observations and likely to be suggestive of examination of the same appearances by other microscopical observers." The general form of the blood-vesicles of the Perameles is the usual circular flattened disc: they presented a greater variety of size than in the Daysurus, but upon the whole a larger average diameter, viz. of an English inch.
Phalangista Vulpiva.—Average diameter of blood-disc 'th inch.
Petaurus sciureus.—Ditto ditto inch.
illacropus penicillatus.—Do. do. inch.
Macropus major.—Ditto ditto inch.
Phascolontys Vombatus.—Do. do. „Leh inch.
The results of the present observations on the blood of the Marsupial quadrupeds cor respond generally with those obtained from the placental Mammalia, inasmuch as the blood discs of the species which derives its nutriment from the greatest variety of organized substances, as the Peratneles, which subsists on insects, worms, and the farinaceous and succulent ve getables, are _larger than those of the strictly carnivorous Dasyure, and of the herbivorous Kangaroo, the blood-discs of the latter, like those of the placental Ruminant, being the smallest, though not in the same proportion. In each natural group of Marsupialia there is a direct relation between the size of the blood disc and that of the species.
&art.—The heart is inclosed in a pericar dium, and situated in the same relation to the lungs, mediastinum, and thoracic cavity as in the Rodent and most other mammiferous quad rupeds. It offers no peculiarity in its general
outward form. The apex is less obtuse in some species, as the Phalanger and Wombat, than in others, as the Kangaroo. The serous layer of the pericardium is reflected upon the large vessels near to the heart. The fibrous layer of the pericardium adheres to the sternum in the Kangaroo. The appendix of the right auricle is always divided into two angular pro cesses, (a, a, figs. 131 and 132,) one in front and the other behind the trunk of the aorta.
Besides this characteristic modification of its external form, the right auricle presents sonic still more essentially marsupial conditions in its interior. There is no trace, for example, of a fossa ovalis' or an annulus ovalis' in any marsupial animal ;* and the absence of these structures, which are present in the heart of all the placental Mammalia, doubtless re lates to the very brief period during which the auricles intercommunicate in the Marsupials, and to the minute size, and in other respects incompletely developed state, at which the young marsupial animal respires air by the lungs, and has the mature condition of the pulmonary circulation established. The right and left auricles intercommunicate by an oblique fissure in the uterine embryo of the Kangaroo, when two-thirds of the period of gestation is past, but every trace of this foetal structure is obliterated in the subsequent growth of the heart ; so that in the mature animal the wide terminal orifice of the posterior cava is separated from that of the anterior cava by a simple crescentic ridge (e, figs. 131 and 132), which forms a salient angle of the parietes of the auricle between these apertures. The an terior cava ( 1)) returns the blood from the right side of the head and the right anterior extre mity; the corresponding vein on the left side (c) passes down in all the Marsupials, as in Birds and Reptiles, behind the left auricle, below the two pulmonary veins, and, after receiving the coronary vein, joins the inferior cava (d) immediately before its expansion into the auricle.
the base of two of the columns is situated at the angle between the septum and the thin outer wall of the ventricle.
The right ventricle extends nearly to the apex of the heart in the Wombat, but falls short of that part in the Kangaroo. The ven tricle is continued in the form of a pyramidal process, somewhat resembling a bulbus arte rinsus, to the origin of the pulmonary artery (f; figs. 131 and 132), and projects beyond the general surface of the heart further than in ordinary Mammalia.
The appendix of the left auricle is notched in the Kangaroo to receive the apex of this pro cess, but not in the Wombat. Two pulmonary veins terminate close together, or by a single trunk, at the upper and dextral angle of this auricle. The mitral valve is regulated by two short and thick columns (k, k,fig.133), which send their tendinous chords to the margin and ventricular surface of the valve.