The _lateral_ surfaces. The terms _upper_ surface and _lower_ surface have fixed values, no matter how they may be approached by the observer. This is not the case, however, with the lateral surfaces, as will appear from the following considerations. Viewing the band as a separate form—as it was spoken of during the life of the twins—we will see that the terms front (“anterior”) and back (“posterior”), as given to the lateral surfaces, were derived from studying the acquired position. Thus we were bound not to cut the “front” of the band, but allowed to make an incision on the “back.” Now this position of selection was destroyed, and its terms deprived of what meaning they may have had, by the reproduction of the congenital relations of the bodies. There is no doubt that in infancy and early childhood there was no acquired position, and, therefore, neither “front” nor “back” to the band. And later, when, as we have reason to believe, the position of selection was gradually adopted, the terms “front” and “back” were reversible—the “front” meaning that which corresponded to the surface of least thoracic approximation. Thus when the adult condition was fixed, and the “front” answered to the widely separated right side of Eng’s chest and left side of Chang’s chest, the “back” was in relation with the closely approximated left side of Eng’s chest and right side of Chang’s. To avoid awkward repetition of phrases expressing the facts of the last sentence, the following characters will be employed in describing the “lateral” surfaces of the band. E. R., C. L. (Eng’s right, Chang’s left) will designate the “anterior” surface of the acquired position. C. R., E. L. (Chang’s right, Eng’s left) will designate the “posterior” surface. Since the right side of Chang’s half of the band merged into the left on Eng’s half, while the right side of Eng’s half, after it passed the middle line, became the left half of Chang’s, we propose using the characters E. L., E. R., and C. R., C. L., which will be understood as signifying left side Eng, right side Eng, etc. Using the above signs we found that the surface E. R., C. L. was inclined decidedly downward and backward when seen in the congenital position, and was 3 in. high and 2-1/2 in. wide. At E. R., the border answering to the ensiform cartilage was marked by a large rounded tubercle; it was much more pronounced than on the corresponding border of C. L. When seen in the acquired position, E. R., C. L. became “anterior,” when, at its upper margin, C. L. was longer than E. R. by one-half inch. The tubercle on E. R., already noticed, was much more prominent than C. L. The contour of the inferior margin was also different, being more uneven. C. L. was not only longer, but was more obliquely placed downward and outward to the centre of the band than E. R. (See cast in the Mütter Museum.) III. Coverings of the Band.