The most marked anomaly in the shape of the kidneys when both are present, and the only one which possesses a clinical interest, is that known as the horseshoe kidney, being a more or less complete fusion of the organs of each side in front of the vertebral column and the great vessels. This fusion is usually at the lower end, but may be in the middle or at the upper end. Sometimes there is a portion lying directly in front of the vertebral column so large and thick as to appear almost like a middle lobe or a third kidney. In a few rare instances this portion has formed a pulsating enlargement mistaken for an aortic aneurism or other abdominal tumor. In others compression of the great vessels has given rise to phlebitis, or the abnormal position of the ureters has obstructed the passage of the urine, with the results, as regards the secondary affection of the kidneys, to be described below. {21} These instances are, however, among the curiosities of medicine, and no rule for their diagnosis can be laid down. A horseshoe kidney is usually discovered only after death, and with no special frequency in cases of renal disease. Variations in the number of the kidneys possess this point of practical interest, that diseases affecting a single organ are more dangerous than if another exists which can take upon itself extra duty. Apparent absence of one kidney may be due to atrophy, attended with very small size of the renal vessels; in which case a small mass of connective tissue is found at the upper end of the ureter, which is usually illy developed. The other kidney is usually hypertrophied. The kidney may fail to be developed. In this case there are no vessels corresponding to the renal artery and vein, and the ureter is stated to be invariably absent, but the writer has seen a specimen where the left ureter terminated superiorly in a rounded cul-de-sac, no kidney or suprarenal capsule being present. The other kidney was of rather large size in proportion to the size of the patient, but of the usual form. This defect is apt to be associated with some anomaly of the genital organs. Another condition, apparently similar, but really due to a fusion of the two embryonic kidneys, is sometimes found. In this the single organ, situated upon one side, is irregular in form and in the number and origin of its vessels. There are usually two ureters, arising one above or beside the other, and directed to their proper positions in the floor of the bladder. A single ureter arising from a single kidney has been seen to empty upon the opposite side of the bladder. Supernumerary kidneys have been noted. In one case an extra pair, situated below the others, were intensely inflamed, while the normal organs were not so.