OCR Text |
Show 1034 MR. W. P. PYCRAFT ON THE [Dec. 19, These last bear only a roughened diapophysial surface on the neuroid. There is no indication of a pleurapophysial element (sacral rib). If these two vertebrae are really sacral then they lie more caudad than usual, being behind the acetabulum and directly opposite the middle of the ilio-ischiadic foramen. The characteristic lumbar enlargement lies between the 2nd lumbar in front and the 1st sacral behind. The free caudal vertebrae vary from 6-7 in number, including the pygostyle. They are very feebly developed in the Grebes. Intercentra occur below the caudal vertebrae both in Colymbi and Podicipides, but are reduced to mere vestiges in the former. The lateral compression of the synsacrum is less marked in the nestling than in the adult; and the high neural crest of the adult preacetabular region is wanting in the nestling. v. THE RIBS. The anterior anchylosed cervical ribs in the Pygopodes, in their form and position, recall those of Phalacrocorax. In the adult they are completely fused above with a downgrowth from the ventral surface of the anterior zygapophysis and below with the anterior and ventral borders of the catapophyses so as to form a bony canal for the vertebral artery. In the Grebes they are found only from the 2nd to the 9th vertebrae, and are comparatively feeble, though long; those of the 2nd vertebrae are mere vestiges. In the Divers (Colymbi) they start from the 3rd vertebra, but terminate, as a pair of vestigial processes on the 10th or 11th ; they differ markedly from those in the Grebes by their great length and thickness, extending backwards so as to embrace the catapophyses of the vertebra next behind, when the neck is straightened out. The posterior free cervical ribs in the Podicipides are two in number (see next page). The penultimate, borne by the 21st vertebra, is long, styloid, and without an uncinate ; that of the 22nd vertebra is longer, extending down to the level of the top of the sternal rib immediately behind it. It bears a large uncinate, but no sternal segment. In the Colymbi there is only one free cervical, apparently corresponding to the antepenultimate rib of the Grebe. The thoracic ribs in the Podicipides are 7 in number, the last two being overlapped by the ilium. 1-5, like the last cervical, bear large uncinates ; these are absent on the 6th and 7th. There are 8 pairs of sternal ribs, the 8th beiug bound by membrane to the posterior border of the 7th. Thus there is evidence of the loss of at least one pair of thoracic ribs. It should be remarked, by the way, that the 7th pair of sternal ribs do not articulate with the sternum. The thoracic ribs in the Colymbi number 8 pairs, all but the last of which articulate with the sternum. The last 3 pairs are overlapped by the preacetabular ilium. The 8th pair are mere vestiges. |