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Show 432 MR. J. W. HULKE ON THE SKELETAL [Nov. 20, may be) lies in a deeper plane in the slab of rock, it is more distant from the observer, as would naturally occur were the surfaces, and not the median edges only, inclined towards each other. Taf. i. fig. 2., Taf. iv. fig. 5 show this point. This hint receives confirmation from Zittel's very instructive plate of a specimen of Pterodac-tylus suevicus from Nusplingen (42). In this is displayed the left half of the pelvic girdle (seen in side view), showing the three pelvic bones still maintaining their normal relations, all contributing to form tbe acetabulum. The ilium and ischium are apparently entire, but the os pubis, in form of a narrow bar, ends abruptly, as if by fracture, at a short distance below the acetabulum. In front of the pubic piece is seen a paddle-like or fan-like piece, which is obviously the part regarded by some authors as prcepubis. The close proximity of this to the part denoted to be pubis by its relation to the acetabulum and the correspondence of its stalk-like end to the apparently fractured end of this suggest that the paddle-like piece originally formed part of the pubic bone. The probability of this view finds strong confirmation in H. v. Meyer's figure of Pterodactylus micronyx (op. cit. Taf. iv. fig. 5), in which the two portions of the os pubis, as I incline to regard them, are shown in their normal connection, a slight apparent break of continuity in the pubic bar marking the point where the paddle-like portion usually becomes detached. W h y should the separation of the two parts of the os pubis so commonly occur at this point ? The form of the pubis in Rhamphorhynchus may elucidate this. The os pubis in this genus has the form of a flattened bar bent angularly near its middle ; one limb of it passes from the acetabulum downwards and forwards in an approximately vertical plane, roughly parallel to that laid through the median axial plane of the pelvis ; whilst the other limb, passing transversely to this axis, meets the corresponding limb of the os pubis of the other side, and unites with it in a median symphysis (42). It is manifest that such an angular bend in the direction of its long axis would be a weak point in the construction of the pubic bar, and would favour its fracture at this point, under stresses acting in any other direction than perpendicular to the plane which contains both the limbs. In Dimorphodon, another genus, the evidence as yet available is not opposed to the idea that its pubis is constructed on a similar plan to that of Rhamphorhynchus, only the large foramen present in this latter between the pubis and the ischium is in Dimorphodon reduced to a narrow cleft. The larger of the two bones, marked 64 in R. Owen's figure of Dimorphodon, and identified by him as pubis (praepubis), may with probability be regarded as the right pubis detached from its normal connections aud displaced, the left pubis lying in advance of the ischium, from which it is separated by a very narrow interval. If, then, in Ornithosauria the bone frequently termed the praepubis is not such but only a detached part of a pubis of a common Lacertilian plan, no corroboration can be found in it that the Crocodilian bone in question is a praepubis. |