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Show 1 9 0 4 .] IN' LIZARDS OF THE GENUS TILIQUA. 155 these ribs bear an extraordinary resemblance to the so-called ‘ abdominal ribs ' of other reptiles," it is particularly to be regretted that the term has been used in so authoritative a work as the Catalogue to which reference lias been made. Since this confusion has been quite unnecessarily introduced, it will be as well to adopt the word " parasternum," already used by Fiirbringer, Gadow, and others. The Lacertilia are at least generally supposed to be without a parasternum, which is one of the points of difl'erence used to distinguish them from the genus Sphenodon. The above quotation from Dr. Gadow implies this general view, which is more explicitly stated in the ‘ Royal Natural History' *. I can find no statement in such works as that section of Bronn's ‘ Tliier-Reich ' which is devoted to the Lacertilia, as well as in other textbooks, to the effect that a parasternum is to be found in the Lacertilia; and I am therefore free to conclude that the knowledge of its actual occurrence is at least not widely spread. In a brief preliminary note in ‘ Nature ' t I pointed out the existence of a series of abdominal ribs in the Scincoid Lacertilian Tiliqua scincoides, and I herewith submit to the Society a more detailed account of these structures (text-fig. 29, p. 156), which I have not up to the present succeeded in observing in any other lizard. The chevron 1 tones of the abdominal-rib system are thin and not always easy to see; their slender bulk, as it appears to me, fully accounts for the fact that they have been previously overlooked. They are not nearly so stout, so numerous, or so closely adpressed as these bones are in an example of Hatteria of about the same size as the two specimens of Tiliqua scincoides which I have examined. For these reasons the bones would very readily be lost in preparing skeletons of Tiliqua. The distance separating the chevrons in Tiliqua is 8 mm., when the abdominal muscles are gently stretched but not overstretched; the same distance in the case of the dried abdominal skeleton of Hatteria was not more than 4 or 5 millimetres. In my specimen of Hatteria there were quite twenty of the chevrons; I could not detect more than seven in one of the two specimens of Tiliqua. In the other specimen these abdominal ribs were hardly at all obvious. The fact that these chevrons are quite independent of the ribs - and their purely superficial position, lying as they do in the ventralmost sheet of muscle of the abdominal wall-is in favour of regarding them as the homologues of the abdominal ribs of Hatteria. It remains, however, to be shown that they overlap the true ribs as the abdominal ribs do in Hatteria. The ribs m Tiliqua after the sternum do not apparently reach so far ventrally as they do in Hatteria. It might therefore conceivably be held that we had here to do merely with the ventral moieties of ribs which were defective laterally, and that the condition occurring * " Another important feature in which the Order [Squamata] differs from all the preceding ones is the absence of any system of true abdominal ribs or ot then-equivalent a plastron " (vol. v. p. 107). f May 5th, 1901, p. 6. |