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Show 246 ON THE ANATOMY OF THE CONDOR. [Apr. 15, far from the end of the complete half of the valve. The seam, as I call it, has hardly a muscular or tendinous appearance. It is not perceptibly raised above the muscle which forms the wall of the heart; it is only conspicuous through its decided white colour. This description, I should observe, applies to the heart after preservation in spirit. I received the whole bird from Australia in spirit some years since. But the seam is so marked, that I cannot but think that it would have been as obvious in the fresh heart as it is in that preserved in spirit. The appearance of the seam, in fact, suggests a thickening of the lining-membrane of the heart, the endocardium. It just runs on to the commencement of the left-hand piece of the muscular existing valve. Now it appears to m e to be fair to construe this structure as a remnant of the otherwise chiefly missing septal flap of the atrio-ventricular valve. It may be admitted that its course is straighter than such a flap had when fully developed. But with rudimentary structures, alterations of one kind or another not related to their former functions are not uncommon. I do not, in fact, think that the length of the seam is against m y interpretation of its nature. As to the possibility that it is a thickening of the endocardium, it seems to m e that it is then very comparable to the "fold" described by Gegenbaur, "which is formed by a thickening of the endocardium." And Gegenbaur adds to the description that the fold in question arises " from the anterior origin of the muscular valve on the septum ventriculorum," which is precisely the origin that the seam described here by myself has. Gegenbaur's fold, however, runs " obliquely backwards and downwards," so that its position as a rudiment is more in accord with that interpretation. A final point is of some little interest. It is or has been believed that ontogenetically as well as phylo-genetically the muscular or tendinous valves of higher vertebrates are first formed as simple thickenings of the endocardium, later invaded by muscle which itself later on is converted into tendon (as in higher mammals). The return, so to speak, of this rudiment of the septal half of the valve to its very earliest condition is worth emphasizing. The facts that have just been dealt with raise another interesting question. At one time the descent of Birds from some Dinosaurian form was widely believed in ; later this view lost some ground, until quite recently Prof. Osborn has recommended its serious reconsideration mainly on the grounds of the discovery of a fourth toe bent backwards, which has been shown to exist in the Dinosauria. This and some other features have added not a little to the bird-like characteristics of that group of Reptiles. On the other hand, there have not been wanting those who would assign the origin of Birds to a lower type of Reptile. The nature of the heart-valves seems to m e to throw some light upon the question. At first sight, the arrangement of the auriculo-ventricular valves in the bird is more suggestive of the same valves in the tortoise than in the crocodile, the latter repre- |