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Show 1870.] AXIAL SKELETON OF THE URODELA. 273 the rib beyond the point from which it starts (figs. 11 & 12). In this case the rib may be truly said to bifurcate distally. Lateral view of sixth vertebra of Salamandra (No. 589 B in Museum of College of Surgeons), showing rib bifurcating at each end. c. Capitular process, t. Tubercular process. A similar process is also sometimes developed from the same part of the ribs next succeeding; but it is rarely to be traced beyond the fourth pair of ribs, and diminishes in size as we proceed from before backwards, and in many forms is not to be detected at all, as far as I have observed, e. g. Menopoma, Cryptobranchus, Menobranchus, Aneides, Spelerpes rubra, and Plethodon. Though, as has been said, the ribs of the opposite side are never connected together by hard parts, but only by membranous prolongations, yet in the middle line of the anterior part of the body there is in many Urodela a solid structure answering to the sternum of higher animals, and connected with the membranous prolongations of the ribs. THE STERNUM. This part is a constant structure in adult Urodela, except in Proteus, Menobranchus, and perhaps Siren. It is rhomboidal in shape, about as broad as long, and with an apex turned forwards. Sometimes, e. g. Salamandra, there is a short xiphoid process, which extends backwards from the middle of its hinder margin. Rarely (e. g. Axolotl) that margin is medianly notched. Each side of the sternum is more or less deeply grooved for the reception of the coracoid lamella, and the inner lip of each groove is much more developed than the outer one. The sternum never ossifies in any Urodele, and originally it is always formed within the coracoids. This might be expected to be the case from the fact that the sternum is a portion of the paraxial skeletal system which the pectoral girdle externally surrounds. But Mr. Parker* has actually verified by observation this primitive condition of the sternum, and proved that the lateral parts of the structure, which embrace the coracoid lamellae externally, are subsequent and secondary outgrowths of a structure which is at first completely internal to the shoulder-girdle. These secondary growths are so large that ultimately the sternum comes to lie outside the coracoids f. * See ' Shoulder-girdle,' p. 65. t Parker, I.e. pl. 3. fig. 14. |