OCR Text |
Show 1036 MR. W. P. PYCRAFT ON THE [DeC. 19, Podicipes jluuiatilis, for instance, the width across the posterior lateral processes may equal the whole length of the corpus sterni; iu other words, the sternum may be as broad as long. In the Divers the width across the widest part is about one-third the total length of the sternum. There is no spina externa nor interna; instead, this region of the sternum is deeply hollowed. The lower lip of the coracoid groove is very large, making the groove exceedingly deep. In the Diver this lip is not greatly developed. The metasternum is deeply notched and not produced backwards into a linguiform plate as in the Divers. The anterior lateral processes are larger and project forward. In the Diver they are sharply truncated, the free anterior border sloping distinctly backwards. The coracoid is short and straight, both in Grebes and Divers. In the former, the epicoracoid is marked by a wide articular surface running transversely across its ventral aspect. The procoracoid process is absent. In the latter the broad articular surface is absent on the ventral aspect aud there is a small procoracoid process. In both there is a well-marked processus lateralis. There is no supracoracoid foramen, as in the Alcidae; the posterior free border of the epicoracoid is almost knife-like and not, as in the Alcidae, squarely truncate. The scapula, as compared with that of the Alcidae, is relatively short, and has but a very narrow transverse articular surface, instead of a very wide one as in Alcidae. In the Podicipides there is a well-marked acromion process projecting downwards from the shaft beyond the level of the coracoid articular surface. The clavicle is not provided with an external lateral facet for articulation with the coracoid, as in many Steganopodes and Alcidae. There is a small hypocleideum. The right and left limbs of the clavicle are very broad and laterally compressed in the Divers. In the Grebes the upper free end of each limb is pointed, and runs along the antero-internal border of the scapula. vii. THE PELVIC GIRDLE. The form of the pelvic girdle in the Pygopodes is unique amongst living birds. Its most characteristic feature is the extraordinary elongation and lateral compression which has taken place. Although the synsacrum has been involved in this compression, it is not, at first sight, so marked as in the innominate bones. The preacetabular ilium is small and narrow, and widely separated from its fellow of tbe opposite side, but is not otherwise very remarkable. The postacetabular ilium, however, takes the form of a broad, flat, almost or quite vertical lamina. This in the Grebe meets its fellow of the opposite side, in the Diver is separated by the knife-like ridge formed by the neural spines of the anchylosed synsacral vertebrae. The ilio-ischiadic foramen is moderately large; the obturator forameu in the Colymbi remains permanently in connection with the fissure of that name, in the Podicipides the |