OCR Text |
Show 1899.] OSTEOLOGY OF THE PYGOPODES. 1035 It is probable that the long styloid free rib of the last cervical or cervico-thoracic vertebra was originally larger and connected with the sternum by means of a sternal rib; in other words, this represents a thoracic vertebra which has been transferred to the cervical series by the loss of the sternal segments and its ribs. Thus, what is now the first was earlier the second thoracic vertebra and rib. In the Podicipides this transference of vertebra from the thoracic to the cervical series is still more marked, inasmuch as wdiat now forms the first thoracic vertebra and rib in the Diver is in the Grebe the last cervical. This seems the most satisfactory way of explaining the presence of the long free ribs in both Diver and Grebe, and wherever else they occur. The transference of the 2nd pair in the Grebe seems to have been comparatively recent, inasmuch as the uncinate is still retained. If this interpretation be correct, and it is one which was, I believe, originally put forward by the late Prof. T. J. Parker, then one more thoracic segment is represented in the Grebe than in the Diver, inasmuch as what now answers to the 1st thoracic of the Grebe really represents the 3rd, and what now answers to the 6th-the last vertebra now connected with the sternum by a sternal rib- represents the 8th thoracic vertebra. The 6th and 7th vertebrae have already fused with the synsacrum. The 7th ( = 9th) ceases to be connected with the sternum, and the rib of the 8th ( = 10th) vertebra is represented only by its sternal segment. In the Diver there is only evidence for 9 thoracic vertebrae, the 9th now venturing but a minute stylet partly fused with the preacetabular ilium, and projecting from its ventral border as a small spine. The presence of these free ribs is exceedingly interesting, they form one of the many links in the chain of evidence, hinted at by Mr. Beddard (1), which goes to show that a shortening of the sternum has taken place. The ribs and uncinate in both Grebes and Divers are relatively broad and flat and of moderate length. In all these particulars they differ markedly from the Alcidae, to which the Pygopodes bear a superficial resemblance. In this last group the sternal and thoracic ribs are of great length. Especially is this the case with the hindmost ribs, which are of enormous length, extending backwards so as to project beyond the level of the free end of the pubes. vi. T H E S T E R N U M A N D P E C T O R A L GIRDLE. The sternum of the Colymbi is very long and bears a superficial resemblance to that of some Alcidae. It can be immediately distinguished therefrom amongst other things by the shallower carina, the feebly developed spina externa, and the large linguiform metasternum, which projects considerably beyond the posterior lateral processes. The sternum of the Podicipides differs very markedly from that of the Colymbi. In the first place, it is conspicuously shorter. In |