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Show 1906.] ON THE SKULL OF A HORSE SHOWING PREORBITAL PITS. 377 Tiliqua scincoides, in which species I have recorded * the structure in question as I believe for the first time; for the elements are larger and more thoroughly chondrified, and thus more easily distinguishable from the tendinous intersections of the abdominal muscles in which they lie than in Tiliqua, as will be seen in the figure (text-fig. 84). When the outermost layer of the abdominal musculature is raised from the deeper layer, the abdominal ribs are raised with the former and can thus be seen to overlie the true ribs which occur in the deeper layer of the ventral musculature. Three pairs of abdominal ribs meet in the middle line and thus form a series of three chevrons. The first two of these possess a forwardly-directed process of the triangular plate which forms the region where the two ribs of the pair are fused. Behind these conies one pair of abdominal ribs, which does not-but only just does not-meet in the middle line. A fifth and sixth rudimentary pair exists ; there is a true rudiment on the right side of a seventh abdominal rib. Behind this only the tendinous intersections of the abdominal muscles are visible. In the region of the parasternum the true ribs do not reach the middle line as cartilaginous rods, and, as already mentioned, they are overlapped by the gristly rods of the parasternum. As Prof. Parker has pointed out t, there are five pairs of true ribs attached to the sternum in Trachysaurus. H e does not, however, mention that a pair behind these also meet and fuse in the middle line a little way behind the sternum. These true ribs meet and fuse superficially and exactly resemble the succeeding abdominal ribs, so far as the median region is concerned. This, however, can invalidate no homology, for the exposure of a true additional piece of xiphisternum is simply due to the absence of pectoral muscles; and in any case the remaining pieces of cartilage so entirely overlap so considerable a portion of the true ribs that they cannot possibly be regarded as the equivalent of their median ventral extremities, which, indeed, themselves reach to within a millimetre or two of the ventral middle line. Mr. R. I. Pocock, F.Z.S., exhibited the skull of a Horse to show the preorbital pit, and made some remarks upon the occurrence of this feature in the skulls of extinct and existing Equidae, and commented on its supposed homology to the prseorbital pit of Hipparion and upon the systematic value that has been attached to it. The following papers were read :- * "On the Presence of Parasternum in . . . Tiliqua, &c," P. Z. S. 1904, vol. p. 154. f Monograph on Shoulder-Girdle, Ray Soc. 1868 p. 114. |