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Show 380 ON T H E D E N T I T I O N O F T H E A R M A D I L L O S . [June 11, 4. In a skull (No. 911B, Brit. Mus.) rather smaller though more mature than the last, the seven milk-teeth described in the last specimen are in place and much worn at their summits ; there is also a small cylindrical posterior or eighth tooth in each side of each jaw. Of the milk-teeth, the first is the smallest, being much compressed, the second is rather broader, the remainder considerably broader and nearly equal. The upper ones are pointed, being bevelled in front and behind. The lower ones are worn mostly on the outside, and have a much more acute apex than the upper ones. The fangs of all are closed at the base, and more or less hollowed by absorption on the inner side, this absorption proceeding sometimes so far as to give a two-rooted appearance to the tooth. At the bottom of each alveolus (as is easily seen when the milk-tooth is removed), and at its inner side (in both jaws), is a distinct recess, in which lies the calcified conical apex of the germ of the permanent tooth, the outer surface of which lies against the hollowed part of the root of the milk-tooth. These germs are all of nearly equal development. The length of the longest upper milk-tooth, from apex to base, is 0"*22, of the longest lower one 0"*25. The length of the tooth-line in each jaw is 0"*7. The milk-teeth are slightly smaller than those of the adult, but otherwise differ very little from them in appearance. They have, however, the important difference of the closure of the base of the fang, causing arrest of growth. The eighth tooth appears to have no predecessor, and consequently is the only true molar. Professor Gervais's observation supplies the next stage; judging from the size of his figure, the animal must attain almost to the dimensions of the adult before the milk-teeth are finally shed. The appearance of double roots which he describes is due only to the absorption of the middle part of the fang, in consequence of the pressure of the growing permanent tooth, which, as in other Mammalia, is placed, in the early stages of growth, not immediately below, but to the inner side of the milk-tooth. A striking exception has thus been shown to a widely accepted generalization. The question naturally arises, Is this really the only exception, and is the generalization itself a sound one 1 It is most desirable that the teeth of other species of Armadillo should be examined ; but it is a singular circumstance that so insufficient are our public zoological collections, that I have not yet been able to find a single example of the right age to throw any light on this question. All available specimens are either too old or too young. With the exception of the one species above described, all statements in reference to the succession of the teeth of these animals appear to rest upon no sufficient basis of observation. |