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Show 554 PROF. E. R. LANKESTER ON THE HEART [June 20, ing to the septal part of the valve, are smaller in size, and in passing from left to right dwindle, so that at the right of the middle line the chordae are inserted directly into the ventricular wall and not into papillary eminences. Only three distinct papillary eminences can be distinguished in this series. Comparing this arrangement with that found in Man, it is obvious that the seven papillary muscles of the anterior part of the valve correspond to the "great" and the "right" anterior papillary muscles of Man ; whilst the superior series connected with the septal part of the valve correspond to the posterior papillary muscles of Man. But in the Rabbit not only must we consider that the " great" and the " right" papillary muscles are divided and represented each by three separate papillary muscles, but also that the attachment of chordae springing from the extreme left of the left anterior cusp and from the left of the septal cusp in the human heart, are in the Rabbit attached to small papillary elevations of muscular substance, instead of being attached simply to the unraised surface of the ventricular wall. It is an interesting question as to whether the condition found in Man and in most mammals is more primitive than that found in the Rabbit. The fact that the Rodents are lower forms than the Simise might lead us to regard the condition seen in the Rabbit as more primitive ; but the very general conformity of the other Mammalia (including such Marsupials and Edentata as have been examined) to the arrangement found in Man, leads to the supposition that the Rabbit's right cardiac valve is a specialization departing from the earlier type preserved in Man. This conclusion will be found to be confirmed by the facts which we now shall expose in reference to the right cardiac valve of Ornithorhynchus. C. O F O R N I T H O R H Y N C H U S P A R A D O X U S . - T h e right auriculo-ventricular valve of Ornithorhynchus is drawn in two sets of figures, accompanying this paper (Plates XXXIX., XL.), taken from the two female specimens in an excellent state of preservation mentioned above. In both of them the membranous collar which forms the complete valve in other Mammalia is seen to be incompletely developed, and not "entire" as stated by Gegenbaur. Instead of the elliptical auriculo-ventricular orifice being completely fringed by the more or less deeply dependent valve, we find only its anterior margin and a small portion of its posterior or septal margin (the extreme right) thus fringed. The septal or posterior portion of the valve is, in fact, almost entirely wanting in one of m y specimens (fig. 16). This result of m y observations is diametrically in opposition to the statements of Gegenbaur quoted at the commencement of this memoir. The absence of the greater part, or even the whole, of the septal flap at once constitutes a very important difference between the right cardiac valve of Ornithorhynchus and that of any other Mammal which is known. Anterior flap.-The well-developed anterior portion of the membranous valve is triangular in form, and is connected with a very large muscular column, which for convenience may be spoken of as musculus |