Aneurysms have been observed all over the world and in nearly all the larger orders. The London Garden has had an unusually large number to report, the most striking being that described by Seligman in the 1906 Report of the Society, in a tiger thirteen years in captivity which had fourteen sacculations from pea to plum size scattered along the aorta. Even with the number of cases on record and those collected here it would be unwise to draw deductions as to their incidence or as to the possibilities of vascular dilatation in any given order. Horses have aneurysms occasionally, cows and dogs rarely, according to Rievel. I can, however, state that there has not occurred in our experience a large growing pulsating aneurysm in the aortic arch region comparable to the condition so well known in man. The literature to which we have had access gives a definite impression that parasitism of vessel walls is the most important factor in the causation of ectasia, and that simple non-parasitic arteriosclerosis is relatively unimportant. Two of our seven cases seem to have been free of parasites but the notes cannot entirely assure one of this. The distribution of cases at this Garden is found in Table 5. [Illustration: FIG. 6.—PARASITIC ANEURYSM IN THORACIC AORTA. PARADOXURE (PARADOXURUS LEUCOMYSTAX). PARTLY SACCULAR, PARTLY DISSECTING ANEURYSM WITH OPEN THROMBOSIS AS INDICATED BY THE GLASS ROD. PIECES OF WORM FOUND IN WALL. COULD NOT OBTAIN WHOLE SPECIMEN, SPECIES UNDETERMINED. ] A dilatation of the first part of the arch in a seal to a size which might be described as an aneurysm caused us to make such a diagnosis, correctly enough from the size and shape of the vessel but possibly worthy of reconsideration in light of the fact that no damage to the wall was found. At the heart and in the descending arch the diameter measured 4–5 cm., while the first part of the aorta measured 7.5 cm. This great irregularity in width could not be found in other seals albeit this section of the arch is usually a trifle larger than its origin and descending portion. The cava in seals is also large, but in this particular animal it measured 6 cm. across at the liver where there is a normal dilatation. These two spaces are looked upon as normal reservoirs for blood during diving, but the case in question seemed to have excessive “aneurysmoid” enlargements without mural disease. London reports an aneurysm of the aorta in a seal.[17]