On the Development and Morphology of the Marsupial Shoulder-Girdle

Abstract An examination is made of a series of early stages in the development of the shoulder-girdle in the common Phalanger (Trichosurus vulpecula), and of early stages in the development of that of the Ring-tailed Phalanger (Pseudochirus peregrinus), and of the Rock-wallaby (Petrogale penicillata). Though in their main features there is much similarity between the girdles of the different genera, there are certain peculiar features in each. In a Trichosurus fœtus of 8·5 mm., though the main part of the scapula is chondrified, together with the glenoid portion of the coracoid and a portion of the acromion, the rest of the arch, the sternum, and the greater part of the clavicle, are still mesenchymatous. The coracoid can be traced downwards as a fan-like expansion, which meets the first rib and the sternum. Between the coracoid and the clavicle a feebly-developed thin sheet apparently represents the precoracoid (epicoracoid). The clavicle is partly ossified at its upper part, and it is very manifest that there is no cartilaginous basis. In the Trichosurus fœtus, at birth (14 mm.), the scapula is well developed. The acromion is a large process which springs from the anterior border of the blade. The spine is not yet formed, though its basis can be distinctly traced as a membranous structure stretching from the acromion upwards along the outer side of the scapula in its anterior third. There is no cartilaginous basis for this part of the spine. The coracoid is of large size, and its lower part, which is somewhat bulbous, articulates with the sternum and with the first rib.