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Show 550 PROF. E. R. LANKESTER ON THE HEART [Julie 20, obvertitur et insidet margine fixo. Recte a Cuvierio maxima ex parte carnea dicitur, quum nonnisi pars libera, anterior, margine leviter concavo circumscripta, membranacea sit. Fasciculi musculares, ad ipsam et e septo et e pariete antico tendentes ad tres ordines reduci possunt. Inferior, major, e pluribus fasciculis componitur ex septo medio infero ad extremum valvulae inferius abit, anterior ex parietis anterioris parte inferiore recte ad basin valvulae ascendit, ubi cum superiore, ex summitate septi descendente confluit." Owen, in the article Monotremata in Todd's Cyclopaedia, vol. iii. p. 390, describes the valve somewhat differently. He distinguishes two fleshy and two membranous portions-the smaller of the latter, placed near the base of the pulmonary artery, agreeing according to him with the smaller muscular fold of the Cursores, whilst the second larger fleshy mass is homologous with the chief muscular valve of the Bird's heart. Gegenbaur (" Zur vergleichenden Anatomie des Herzens," Jena-ische Zeitschrift, 1866, vol. ii. p. 381) objects to this identification, although he practically admits something very much like it in comparing the valve of Ornithorhynchus to that of the Crocodile, which, in its turn, may be readily shown to have common features with that of the Bird. In his ' Elements of Comparative Anatomy,' English edition, p. 584, Gegenbaur speaks of the fleshy structure of the heart of Ornithorhynchus as being a retention in this animal of a condition which is not unknown in other Mammalia, but is transient in them, being found at an early period of development. In his memoir in the 'Jenaische Zeitschrift,' Gegenbaur gives an original description of the right auriculo-ventricular valve of Ornithorhynchus, but no figure of it. His description does not agree with that of his predecessors, nor with what I have observed. He says:-" I find the entire circumference of the right atrio-ventricular ostium beset by a membranous valve, which has developed muscular bundles only at certain parts, and is disposed, as the following account shows, somewhat otherwise than Meckel and Owen have stated. Two portions may be distinguished in this valve - a part adjacent to the ventricular septum, and a part which fringes the ostium along the outer wall of the ventricle. The two portions posteriorly pass into one another, and anteriorly, in the neighbourhood of the origin of the pulmonary artery (that is, at the conus arteriosus), are separated from one another, inasmuch as here a spot is found in the circumference of the ostium in which the valve is interrupted." [This does not accord with the previous statement as to the " entire " circumference being beset with the valve. As will be seen below, in the hearts examined by m e a very large part of the circumference of the ostium is devoid of any valve.] "The portion of the valve corresponding to the outer ventricular wall begins broadly at the conus arteriosus (or anteriorly) ; it stretches outwards and backwards, becoming broader, and then narrows again and passes into the median division of the valve. At the anterior point of fixation of the valve, two strong muscular bundles pass from the ventricular septum into the valve, and run (the heart being supposed to have its |